Abstract: A thermal-hydraulic microcomputer code (RETAC, REactor Transient Analysis Code) for transient analysis of AP1000 primary loop was developed using FORTRAN language. The loss of flow accident (LOFA) was analyzed using RETAC. Transient characteristics of some main system parameters were obtained, including the maximum fuel temperature and MDNBR in the hot channel, pressure and water level in the pressurizer and steam generator secondary side. The results show that at the early stage of the loss of flow accident, the highest fuel central temperature and MDNBR in the hot channel do not exceed specified limits and meet the safety criteria. Modular programming technique was adopted for RETAC and it is convenient for further modification and application. It is expected that the present work is instructive to develop Chinese own independent software for the design and safety analysis of large scale PWR.