The study presents the combined effects of using nanofluid and varying fin cross-sectional shape on the heat transfer characteristics of a micro pin-fin heat sink by employing discrete phase model (DAM). Three fins configurations of the square, circular and hexagon cross-section with constant fin diameter and height have been analyzed for the inline arrangement of 17 x 34 fins. Aqueous nanofluid containing spherical shaped particle dispersions of TiO2 has been simulated for the particle concentration and size of 4.31 vol% and 30 nm respectively. Constant heat flux (192 W) boundary condition at the base of heat sink has been considered for the range of Reynolds number 250 <= Re <= 550. The influence of fin shape on the thermal efficiency of the heat sink has been analyzed by evaluating heat sink base temperature, Nusselt number, convective heat transfer coefficient distribution and temperature contours along the surface of the heat sink. Additionally the velocity streamlines and contours have also displayed to elaborate the fluid flow attributes. Results demonstrate that under identical flow conditions, the nanofluid cooled circular fins displayed most efficient thermal performance followed by the hexagon and square fins. While the water cooled square fins depicted lowest heat transfers characteristics. The best thermal performance of the circular fins is the response of the delayed flow separation along the smooth surface of fins and the subsequent uniform flow distribution along the whole sink. For all the cases, upstream fin rows played a primary contribution in flow distribution and hence thermal characteristics of the heat sink. (C) 2018 Elsevier Ltd. All rights reserved.