Abstract: The effect of hydraulic equivalent diameter,thermal equivalent diameter,characteristic distance,and water properties on heat transfer and flow characteristics were numerically analyzed for supercritical water in circular tube and annular channel.The calculation was performed using FLUENT in r-z two dimensions.The appropriate mesh layout was determined based on mesh sensitivity analysis and SST model was taken as the best turbulence model for supercritical water in this work.The results show that the effect of thermal equivalent diameter can be neglected,while hydraulic equivalent diameter,characteristic distance and water properties are very important for heat transfer and flow characteristics of supercritical water.