Abstract: The nuclear power plant and passive residual heat removal system were modeled and investigated with the advanced code developed from RELAP5/MOD3.2 code, for analyzing the influence of local natural circulation on the passive residual heat removal in nuclear reactor. The simulation results were validated with experimental data. The results indicate that the local natural circulation occurs in the connected region consists of reactor radiation zone, upper head, annular channel and lower plenum in case of natural circulation operation, due to the upper head by passing flow and the leak flow at the core inlet. As the natural circulation capability is weak, the local natural circulation in the core is so significant that the decay heat can not be removed successfully.