A novel method was proposed for the evaluation of Mode I dynamic fracture tough- ness(DFT)under plane stress and small scale yielding conditions for welded joints of stainless steel(SS),OCr18Ni10Ti.In a hybrid experimental-numerical approach, the experiments were carried out on the Hopkinson pressure bar apparatus,and three dimensional(3D)transient numerical simulations were performed by a finite element (FE)computer program.Macroscopical plastic deformation was observed at the load- ing and supporting points,on the specimens,after the test,which could cause a large error if omitted in the numerical simulation.Therefore,elastic-viscoplastic analysis was performed on the specimen by adopting the Johnson-Cook(J-C)model to describe the rate-dependent plastic flow behavior of the material.The material heterogeneity in the mismatched welded joints,induced by the difference in the base metal(BM)and the weld metal(WM)in yield stress,has also been taken into consideration by using the J-C models separately.Good accordance was obtained between the experimental and the computational results by the present approach.The relationship between plane stress DFT and loading rate was also obtained on the order of 10~6 MPa.m~(1/2).s~(-1). A novel method was proposed for the evaluation of Mode I dynamic fracture tough-ness (DFT) under plane stress and small scale yielding conditions for welded joints of stainless steel (SS), OCr18Ni10Ti. A hybrid experimental-numerical approach, the experiments were carried out on the Hopkinson pressure bar apparatus, and three dimensional (3D) transient numerical simulations were performed by a finite element (FE) computer program. Macroscopical plastic deformation was observed at the load- ing and supporting points, on the specimens, after the test, which could cause a large error if omitted in the numerical simulation. Beforefore, elastic-viscoplastic analysis was performed on the specimen by adopting the Johnson-Cook (JC) model to describe the rate-dependent plastic flow behavior of the material. material heterogeneity in the mismatched welded joints, induced by the difference in the base metal (BM) and the weld metal (WM) in yield stress, has also been taken into consideration by using the JC mode ls separately. Good was obtained between the experimental and the computational results by the present approach. The relationship between plane stress DFT and loading rate was also obtained on the order of 10 ~ 6 MPa.m ~ (1/2) .s ~ (-1).