Abstract: Under the condition of natural circulation in reactor systems, reflux may occur at U-tubes of steam generator(SG). Taken a SG of a Generation III Pressurized Water Reactor (PWR) with passive safety systems as the prototype, the thermal-hydraulic characteristics of the SG reflux are investigated. In this paper, the SG U-tubes are divided into six groups according to their lengths, and the flow and heat transfer characteristics of single-phase fluid in the U-tubes are analyzed with (CFD) methodology. The trends of total pressure drop against mass flow rate are derived based on the simulation results. The effect of the U-tube length and the temperature difference between the primary side fluid and the secondary side wall(?T) on the reflux are discussed. It is shown that when ?T is constant, the reflux occurs more often in the longer U-tubes. When the U-tubes length is constant, the smaller ?T is, the more often reflux occurs.