Convective heat transfer using nanofluid in microchannel heat sink has been performed numerically. Rectangular, semi-circular, triangular and trapezoidal cross section shapes of microchannel heat sink are employed in this present study. ANSYS Workbench 15.0 is set up to solve three-dimensional laminar flow of fluid and heat transfer in microchannel heat sink using the fluid flow system in ANSYS FLUENT. The working fluid used is Al2O3/water nanofluid with nanoparticle concentrations in the range of (0%-5%). All microchannel heat sink walls are set as adiabatic walls, while the heated plate is set at constant temperature of 340K. Uniform velocity and 300K constant temperature are considered at inlet. Effect of the four cross-sectional shapes on the behavior of heat transfer and Darcy friction factor is studied at Reynolds number is in the range (100-900). Average and local Nusselt number and Darcy friction factor are calculated. The effect of nanofluid concentration on Nusselt number, Darcy friction factor and thermal-hydraulic behavior is studied. Similarity between the results found in the present study and results found in literature is apparent.