Abstract: We deal with a proposed natural circulation system under atmospheric pressure. The codes are developed by us for the purpose of analyzing the performance of the system. The startup, operating process are simulated through the codes. Furthermore, the influence of flashing on the system behaviors is studied. The results show that the system can build up natural circulation quite quickly from beginning during startup phase and can provide reliable natural circulation capacity for long time. It can also be concluded that the flashing of the water is dominant for the system in long run. The sensitive analysis also proves the height difference between heat sink and heat source plays a very little role on the performance of the system.