In this study, the forced convective heat transfer of pure water and alumina-water nanofluid with volume concentration of 0.5% and 1%, as a cooling fluid through a microchannel heat exchanger was experimentally investigated. This microchannel consists of 17 parallel channels with a rectangular cross section with 400 µm width, 560 µm height and 50 mm length. The experiments were performed in the range 600 to 1800 of Reynolds and constant heat flux conditions (19 W/cm2). Stability studies showed that alumina-water nanofluid at pH = 3 for 3 hours in a bath of the ultrasonic vibrating demonstrate the maximum stability. The variations of microchannel surface temperature, fluid temperature at the entrance region of the microchannel, average heat transfer coefficient of nanofluid and pure water, and their friction factor measured experimentally. Also comparison between average Nusselt number with existing heat transfer relationships was performed. The results show that heat transfer using nanofluid shows considerably increase in comparison to water. So that the maximum amount of average heat transfer coefficient for alumina-water nanofluid with 0.5% concentrations is about 32.8% and for alumina-water nanofluid with 1% concentrations about 49.7% in comparison to pure water. It was also found that the heat transfer coefficient increases with increasing Reynolds number and nanoparticle volume fraction.