Abstract: The analysis code for an integrated pressurized water reactor was developed on the basis of best-estimate transient analysis code RELAP5/MOD3 for nuclear power plan. The point reactor kinetics model in the original code was replaced by the two-group, 3-D spatial and temporal neutron kinetic model. The coupling model for three dimensional physics and thermal hydraulic was established. The transition process from forced circulation (FC) to natural circulation (NC) was simulated with this code. During the transition from FC to NC, the variation of reactor power is significant, while the coolant temperature is greatly affected by the coolant flow rate.