A new method, using the fracture-surface topography analysis (FRASTA), of determining the parameters of rotation factor and CTOA is provided. The fracture surfaces are scanned by laser microscope and the height data is recorded to analyze. By comparing the crack extension, plastic rotation factor and COA obtained by the method of double clip gauge and by this new method respectively, the validity of it is verified. It is then used to determine the CTOA. The critical CTOA is well determined in this paper. Also by the new method, the variation of crack opening deformation ( COA and CFOA) through specimen thickness can be obtained which cant be obtained by the usual methods. It also can provide much information of crack extension such as direction and ratio.