Abstract: The heat transfer characteristics in forced circulation in narrow rectangular channels under the operation condition of constant coolant average temperature have been experimentally and theoretically studied.The result shows that the heat transfer temperature difference between wall and water increases linearly with the Reynolds number in the laminar flow region,however,when the flow comes into the turbulent flow region,the heat transfer temperature difference nearly keeps constant with the increase of Reynolds number.The above phenomena were explained through the analysis of heat transfer mechanism.In addition,the larger the temperature differences between inlet and outlet,the smaller the Nusselt number.The reason that the temperature difference has effects on heat transfer characteristics is that the proportion of thermal resistance in the viscosity sub layer has been changed.