Two criteria of ductile fracture strain are suggested. From the theory of plasticity for porous materials, the following criterion of fracture for a triaxial state of stress is obtained: [numerical formula] where ε_e_q_f is the equivalent fracture strain, σ_m the mean stress, σ_e_q the equivalent stress, a₀ and b₀ are constants. Except under certain conditions, this criterion shows reasonable agreement with experiment. To improve the accuracy of the prediction of the fracture strain, the above criterion is modified as follows: [numerical formula] where c₀ is a constant. It is found that this criterion provides a greater accuracy for prediction of the fracture strain.