In order to understand the heat transfer performances of the liquid metal with low melting point compared with conventional coolants, the heat transfer performances of laminar flow in a circular duct with constant wall heat flux using liquid gallium and water as coolants were theoretically analyzed by considering the influence of thermally developing region, and the effect on heat dissipation was evaluated by using the mean wall temperature. The results indicate that when duct length(Z,) or Re remains constant, the critical Re(Re,c) and critical length($Xc$) exist respectively, the cooling effect of liquid gallium is better than that...