Convective Heat transfer of MgO-water nanofluid in a microchannel Heat sink is experimentally investigated in various concentrations of 0. 01, 0. 05, 0. 1, and 0. 6 wt%. The microchannel consisted of 48 parallel rectangular cross section channels with the height of 800 μ m, width of 524 μ m and length of 52 mm. A well stability duration (ca. 1 month) was resulted by a 180 min ultra-sonication of the MgO suspension. The experiments in the microchannel were then performed in a flow rate range of 0. 5 to 2. 2 l/min while the inlet temperature and Heat flux were constant. The results indicated that using the MgO nanofluid in low flow rates and concentration has less effect in improving the Heat transfer coefficient, while it becomes highly efficient by the simultaneous increase of flow rate and concentration. An enhancement of 162. 3% in convective Heat transfer coefficient at the channel inlet was achieved at the concentration of 0. 6 wt% and the flow rate of 2. 2 l/min, and at the same condition, the average Nusselt number also increased up to 52. 8%. However, the nanofluid at 0. 1 wt% was more efficient compared to the other concentrations in increasing Nu at higher Re.