In this study, single-phase laminar flow of deionized water through microchannel heat sinks is investigated experimentally. Heat sinks including twenty-nine parallel rectangular channels have been used as the test samples. Three different channel geometries with different aspect ratio (AR = 1.22, 0.82 and 0.37) but with the same hydraulic diameter (100 gm) are considered. A constant heating power of 9 W is applied to the test section. The range of Reynolds number varies from 12.3 to 47.3. It is disclosed that increasing AR increases Nusselt number while decreasing the friction factor.