The influence of an improved design of one and two fillet profile on the heat transfer rates in heat sink subject to parallel flow is investigated. The investigation of heat sink with fillet profiles has been compared with the heat sink without fillet profile. In this study, water was used as the coolant, and the flow was in the single-phase regime under laminar flow condition at Reynolds numbers ranging from 200 to 1300. The heat sink is made from copper where a constant heat flux was applied at the lower surface of the heat sink. The comparison of hydrodynamic and thermal characteristics of all cases and cooling fluids has been made under the same value of Reynolds number and constant heat flux. The results show that the one fillet profile has a slight effect on pressure drop calculations and the heat transfer rate especially at high Reynold number. However, using two fillet profile has a significant influence on heat transfer characteristics and pressure drop at all rates of Reynold number. The two-fillet profile increases the Nusselt number by 15% compared to smooth microchannel and 25% decrease in pressure drop. Therefore, the heat sink with two fillet profile design has a potential to be used to enhance the thermal performance of cooling system with low pressure drop.