The power consumption of individual electronic devices has increased with the performance improvements in recent years. Accordingly, the heat quantity generated from electronic devices has increased to the extent that the performance is limited by the conventional cooling technology. In this study, a water cooling method with microchannels is utilized to cool electronic devices as represented by a CPU. The test equipment is designed to record the average heat transfer data at the microchannels. The resulting test data illustrates that the heat transfer coefficient depends mainly on heat flux and little on mass flow rate. The present results are also compared to existing correlation equations. Additionally, the influence of hydraulic diameter of the microchannels on heat transfer is clarified.