Microchannels are effective heat sinks for microelectronic systems. However, it remains unclear what form of channels will be most effective in improving the overall performance of microchannel heat sinks. The effect of channel geometry was studied using computational fluid dynamics to understand the heat transfer and fluid flow characteristics of microchannel heat sinks with rectangular grooves in sidewalls and different shaped ribs in the center core flow. Four different rib configurations are considered, including rectangular, diamond, forward triangular, and elliptic. In order to fully understand the design and operation of microchannel heat sinks, the overall performance of these devices was analyzed and evaluated in detail in terms of Nusselt number, apparent friction factor, and thermal enhancement efficiency. The enhancement mechanism of fluid flow and heat transfer was discussed in order to determine the optimum channel structure. The objective was to optimize channel structure for microchannel heat sinks, thereby enhancing heat transfer and reducing flow resistance and pressure drop. The results indicated that the overall performance can be greatly improved by the combination of grooves and ribs. This combination can make full use of the advantages of ribs to increase flow disturbance and to enhance heat transfer, and the advantages of grooves to increase flow area and to reduce pressure drop. The shape and dimensions of ribs have a significant effect on the overall performance. The overall performance obtained with rectangular ribs is the best at Reynolds numbers less than 500, but lower than that obtained with elliptic ribs at Reynolds numbers greater than 500, and even lower than that obtained with diamond ribs at Reynolds numbers greater than 700. The rectangular grooved channel with rectangular ribs yields the best overall performance with a relative rib width of 0.25 and at a minimum rib-groove distance of 0.1 mm and a Reynolds number of 500.