Abstract: Crack leak rate calculation is the foundation of leak before break (LBB) technology application in nuclear power plant pipe system. The crack leak rate calculation results were obtained with isenthalpic flow assumption based on Fauske two-phase critical model with considering the friction effect. A Mathcad code was developed to calculate crack leak rate. According to non-equilibrium two-phase flow theory, the modification of the model taken into account thermodynamic non-equilibrium effect on the crack leak rate was also implemented. All the calculation results were compared with experimental data. The results show that the two-phase crack leak rate decreases with the increase of crack length to diameter ratio L/D, but increases with either the stagnation pressure or the fluid sub-cooling at the crack entrance. The mathematical model can predict consistent trends with experimental data, but the calculated critical flow rates with the model omitting thermodynamic non-equilibrium effect are lower than experimental data. While considering crack flow as thermodynamic non-equilibrium, the agreement between the calculated results and experimental data is very good, showing that the Mathcad code with modified model made in the paper can be used to calculate the crack leak rate accurately and provide application foundation of LBB in nuclear plant pipe system.