A three-dimensional numerical simulation of an interrupted microchannel heat sink is performed to investigate the pressure drop and heat transfer characteristics of a nanofluid flow. In the first part the effect of two different rib shapes on flow characteristics is studied to find the optimum design. In this part water is used as working fluid. For the second part of the present work, an Al2O3/water nanofluid with different volume fractions is modeled as the working fluid, and its performance is compared to water. Analysis shows that using ellipse ribs results in better performance of the microchannel rather than diamond ribs and no ribs. It was also shown that at least 2% of nanoparticles volume fraction is required to achieve an enhancement in heat transfer.