Abstract: Forced convection of the circular microchannel is analyzed. The governing equations of circular microchannel are solved by separation of variables, with the considerations of axis conduction, viscous dissipation, thermal entrance and velocity slip. Then the expressions of temperature field and Nusselt number at uniform wall temperature conditions are achieved. Simulation of heat transfer is conducted. It is shown that the average heat transfer coefficient of microchannel is larger than macrochannel when the radius decreases for the small size effect, which means that face the average heat transfer coefficient of several microchannels on the same surface area is lager than that of one macrochnnel.