Abstract: Numerical simulation on the flow and heat transfer characteristics of sodium flowing in an annuli was conducted using ANSYS CFX. The inside diameter of the annuli was from 4mm to 10 mm while the outside diameter from 7mm to 20 mm. The influence of the velocity, inlet temperature, heat flux from the inner surface and outside surface were analyzed. Calculation results showed that the gap of the annuli was with great effects on the flow and heat transfer characteristics of sodium significantly, but the heat flux with little effect. Results in this work were compared with the theoretical and experimental results from literatures. A correlation of the heater transfer coefficient for liquid sodium flowing in an annuli was proposed based on the analysis. The calculation results in this work show good consistency with other theoretical and experimental results.