The determination of pure shear (Mode Ⅱ) fracture toughness, KⅡc, for brittle rock is studied. Three types of specimens. which are anti-symmetric four-point bending specimen. direct shearing specimen and shear-box specimen. are experimentally investigated. A new criterion is established for either ModeⅠ (tensile) or Mode Ⅱ (shear) fracture of brittle rock, which clearly distinguishes the fracture mode from the loading mode. With guiding-grooves in both the anti-symmetric four-point bending and the direct shearing specimens, Mode Ⅱ fracture can be successfully created. Numerical calculations show that the guiding-grooves in the notch plane can depress the tensile stresses at the notch tips and enable the crack to propagate along the notch plane. The value of K11. obtained in this case is 2-3 times as high as the magnitude of KⅠc, which is considered as the true Mode Ⅱ fracture toughness of rock. Mode Ⅱ fracture can be easily created in a shear-box loaded square specimen. The compressive stress in the two directions can completely depress the tensile stresses around the notch tips and makes it easy for the crack to initiate and propagate along the notch plane, leading to Mode Ⅱ fracture. The shear-box test is suggested as a method for determining the Mode Ⅱ fracture toughness of rock. Both theoretical and laboratory studies of shear-box test show that the apparent Mode Ⅱ fracture toughness KⅡcσ,i.e. Mode Ⅱ fracture toughness under the compressive stress, linearly increases with the compressive stress a- on the notch plane. The intercept of straight line of KⅡcσ versus σ- (i. e. σ = 0) is defined as the Mode Ⅱ fracture toughness KⅡc .In some cases, it is not KⅡc but KⅡc σthat needs to be known. For instance, the pre-existing cracks and discontinuities in rock and rock masses are subjected to compressive loading in most cases. What is useful in this case is the Model Ⅱ fracture toughness under the in-situ compressive stress, KⅡcσinstead of KⅡc . The size effect on the determination of KⅡc has been experimentally studied. The examined dimensions include the specimen thickness B, the dimensionless notch length a/W, and the notch inclination angle a. Test results show that KⅡc decreases with the increment of the specimen thickness B and becomes a constant when B is equal to or larger than the specimen height W for both the single- and double-notched specimens. When the dimensionless notch length a/W increases, KⅡc decreases and approaches a limit if a/W is in the interval of 0.5-0.6 for single-notched specimens and 0.3-0.35 for double-notched specimens. The notch inclination angle has a significant effect on the failure pattern of the specimen. Too small or too large a will lead to either local crushing or Model Ⅰ fracture. The favorable range for a is 60°-75°for Model Ⅱ fracture. Finally, a new criterion is established based on the examination of the Mode I and Model Ⅱ stress intensity factors, K1(θ) and KⅡ(θ). The criterion can predict either Model I or Model Ⅱ fractures, no matter what kind of loading condition is applied. Model Ⅱ fracture occurs when the ratio of KⅡmax to KⅠmax, is larger than that of KⅡc to KⅠc and KⅡmax, reaches its critical value, KⅡc. The determination of pure shear (Mode II) fracture toughness, KⅡc, for brittle rock is studied. Which are anti-symmetric four-point bending specimen. Direct shearing specimen and shear-box specimen. Are experimentally investigated. A The new criterion is established for either Mode I (tensile) or Mode II (shear) fracture of brittle rock, which clearly distinguishes the fracture mode from the loading mode. With guiding-grooves in both the anti-symmetric four-point bending and the direct shearing specimens, Mode Ⅱ fracture can be successfully created. Numerical calculations show that guiding-grooves in the notch plane can depress the tensile stresses at the notch tips and enable the crack to propagate along the notch plane. The value of K11. obtained in this case is 2-3 times as high as the magnitude of KIc, which is considered as true Mode II fracture toughness of rock. Mode II fracture can be easily created in a shear-box loaded square spec The compressive stress in the two directions can completely depress the tensile stresses around the notch tips and makes it easy for the crack to play a notch and the leading to Mode II fracture. The shear-box test is suggested as a Method for determining the Mode Ⅱ fracture toughness of rock. Both theoretical and laboratory studies of shear-box test show that the apparent Mode Ⅱ fracture toughness KⅡcσ, ie Mode Ⅱ fracture toughness under the compressive stress, linearly increases with the compressive stress a-on the notch plane. The intercept of straight line of KⅡcσ versus σ- (ie σ = 0) is defined as the Mode Ⅱ fracture toughness KⅡc. In some cases, it is not KⅡc but KⅡc σthat needs to be known. For instance, the pre-existing cracks and discontinuities in rock and rock masses are compressive loading in most cases. What is useful in this case is the Model II fracture toughness under the in-situ compressive st ress, KⅡcσinstead of KⅡc. The size effect on the determination of KⅡc has been experimentally studied. The inspection dimensions include the specimen thickness B, the dimensionless notch length a / W, and the notch inclination angle a. Test results show that KⅡc decreases with the increment of the specimen thickness B and becomes a constant when B is equal to or larger than the specimen height W for both the single- and double-notched specimens. When dimensionless notch length a / W increases, KⅡc decreases and approaches a limit if a / W is in the interval of 0.5-0.6 for single-notched specimens and 0.3-0.35 for double-notched specimens. The notch inclination angle has a significant effect on the failure pattern of the specimen. Too small or too large a will lead to either local crushing or Model I The new model is established based on the examination of the Mode I and Model II stress intensity factors, K1 (θ) and K Ⅱ (θ). The crit Model II fracture occurs when the ratio of KIImax to KImax, is larger than that of KIIc to KIc and KIImax, reaches its critical value, KIIc .