High performance electronic devices may require heat dissipation as high as 107 W/m2. Such a high heat flux can not be removed from the chips using conventional electronic packaging. Microchannels, having a large surface to volume ratio together with large heat transfer coefficient appear to be potential candidates for cooling electronic chips. The purpose of the present work is to study the effects of various geometric characteristics of the flow channel, fins, and the substrate on the thermal resistance between the substrate and the cooling medium as well as the uniformity of the substrate temperature. The effects of coolant flow rate will also be examined. The three dimensional convection in the channel and conduction in the walls were solved simultaneously using a general purpose CFD software. The results of simulation have shown that: (1) a nonlinear bulk fluid temperature distribution develops along the channel which contributes to a more uniform substrate temperature, (2) increasing the channel (fin) height above a certain value does not decrease the substrate-fluid thermal resistance, and (3) increasing the fin thickness reduces the thermal resistance at the cost of higher bulk fluid temperature.