Abstract: The two-phase reverse flow phenomenon may occur in the inverted U-tubes of steam generator (SG) for natural circulation. A theoretical model of flow and heat transfer for two-phase flow in U-tubes based on the homogenous flow model was established in this paper. A pressure drop-mass flow curve was derived to investigate the mechanisms of two-phase reverse flow in U-tubes. The effects of inlet mass quality and fluid temperature in secondary side on the reverse flow were also analyzed. Then the theoretical results were verified by the RELAP5/MOD 3.3 codes. The research shows that increasing the SG secondary side pressure may reduce reverse flow phenomenon in U-tubes, and the phenomenon more likely occurs when the inlet mass quality increases.