[en] AP 1000 designed by Westinghouse Electric Company has become one of the leaders in the third generation nuclear power plant. More and more researches concentrate on the AP 1000 safety assessment considering the important status of Aaap in China. This paper aims to build the main system model of Aaap plant and study its behavior in case of a loss-of-coolant accident (LOCA) based on the simulation code RELAP5. The model in a steady state makes it possible to perform the desired transient analysis. In the present study, the response of AP 1000 primary circuit to a SB-LOCA in the non-pressurizer side is simulated. Difference between 10-in. cold leg break and 10-in. hot leg break is investigated. The result shows that a hot leg SB-LOCA has the similar effect to a cold leg SB-LOCA but the accident result of the former is less serious. The results of a spectrum of hot leg breaks show that the diminution of RCS pressure is much slower for a smaller break, which leads to different transient behaviors. From the simulation result, transient operating features for AP 1000 under a hot-leg SB-LOCA situation can be concluded. The research data can be beneficial to the prevention and management of a similar accident. (authors)