The Passive Residual Heat Removal System (PRHRS) of the AP600 is designed to remove the residual heat through heat exchangers located in the In-containment Refueling Water Storage Tank (IRWST) when Steam Genarators (S/Gs) lose their heat removal capabilities. In domestic area, PRHRS performance has been analyzed using the simple computer program and RELAP code and a heat transfer test through the heat exchanger tube has been planned with support from the CARR. In this study, the heat removal capability of the PRHRS has been evaluated by simulating the realistic system responses in conjunction with the NSSS and the secondary system. In addition, the effect of steam generators on the PRHRS performance has been investigated. For this study, the system transient analysis code has been developed based on the existing computer code which is capable of analyzing the transient related to the feedwater system and natural circulation in the primary system. The developed computer code simulates system responses for a Loss Of AC Power to the plant auxiliaries (LOAP) quite well and it has been shown that PRHRS satisfies the system design requirements. During the LOAP transient S/Gs are functioning as heat sinks until boiled dry, but, they act as heat sources after the primary system is cooled. Analyzed results for the reactor trip time from 26 to 70 seconds after the loss of normal feedwater by adjusting S/G low level trip setpoints and for the PRHRS actuation delay time from 300 to 1,500 seconds show that there are minor effects on the PRHRS performance for the reactor trip time and for the actuation delay time until 1,200 seconds, respectively.