A heat sink for dissipating heat from an electronic device includes a base for contacting the electronic device. A U-shaped heat pipe is fixed to the base and includes an evaporating portion contacting the base and two condensing portion extending from opposite ends of the evaporating portion and through a plurality of first fins stacked together parallel to each other. A plurality of second fins extends from the base towards the first fins and supports the first fins. A reinforcement is fixed on the base and joins with some of the first fins, thereby enhancing integrity of the first fins.