Abstract: A key issue of SCWR development is the thermal hydraulic characteristics of supercritical water (SCW) flowing in rod bundles. The heat transfer of SCW in a 4-rod bundle was numerically simulated by CFD in this study. The SSG turbulent model was selected and a reasonable prediction was achieved comparing with the experimental data. The simulated results show that the flow direction has great impact on the flow rate distribution at cross section. Comparing with the downward flow, the upward flow has more uniform flow rate distribution and circumferential temperature distribution which results in lower wall temperature, though its flow mixing in the bundle is relatively weak. It indicates that the convective heat transfer on heating wall is mainly influenced by flow rate distribution in the cross-sectional flow area of bundles.