Abstract: During the process of LOCA (loss of coolant accident) in a PWR, the system pressure of primary circuit will decrease and great amount of vapor will be generated. The condensation might be occurred on cold leg then it would flow to downcomer, when the water in safe injection pipe poured into cold leg. In order to explore this phenomenon, the T-junction condensation experiment was carried out. Under the circumstance of pure steam flows in the main pipe and subcooled water flows in the safe injection, the mass flow rate of condensation in different initial conditions was researched. The results indicate that the condensation has a limit, then a model by using condensation efficient and thermodynamic ratio (R_T number) is proposed based on the results.