A new method of fracture toughness K1C prediction of non power law hardening material by using cleavage fracture local approach is proposed in this paper. The fracture toughness of A508-Ⅲ 16MnR at different cleavage fracture probability are predicted by using the method. To most of pressure vessel carbon-manganese steels, cleavage fracture is likely to occur at the load corresponding to 62% cleavage fracture probability. Hence, the fracture toughness corresponding to the load is the most possible fracture toughness of the steels. The values of fracture toughness corresponding to 62% cleavage fracture probability is close to that of testing fracture toughness. The work of this paper expends the application of Beremin cleavage fracture model in predicting fracture toughness.