Abstract: By using RELAP5 and MELCOR, transient behaviors of containment and PCS were investigated. The Influences of various parameters, such as pressure, condenser area and loop height were also studied. The results show that the heat in the containment can be effectively removed by PCS in a period of accident without spray. However the tank water should be made up and cooled for the long run. The heat transfer power of two-phase flow case is always higher, and varies more substantially with pressure, than that of one-phase flow case under the same boundary temperatures. However, flow instability may occur for two-phase flow case. By increasing the condenser area, the heat transfer is enhanced. The influence is less insignificant when the tank temperature is relatively high. It is also found that for both one-phase and two-phase flow, the change of loop height has little effect upon the heat transfer power under the given boundary condition in this study.