Abstract: Based on fatigue constant life curve, the concept and the universal expression of a generalized fracture constant life curve are presented to determined the two-dimensional probability distribution of the fracture threshold value,by victue of the clear physical significance of the expression and the reasonable relation between the mean and the amplitude of the fracture threshold value. With the optimization method of the correlation coefficient, the parameter estimation formulas of the constant life curve are induced,the generalized fracture constant life p-ΔK_th-K_mth curve with the reliability level p is given. From p-ΔK_th-K_mth curve, the two-dimensional probability distribution of the fracture threshold value is derived. Then, LY11 and 40CrNiMoA are respectively tested under three different average stress, the aforementioned method is applied to analyze the test results,and it is found that a high precision is obtained with the above approaches.