A heat sink with a built-in heat pipe for semiconductor package is disclosed. At least a built-in heat pipe is disposed in a cavity of a metal vessel and sealed by a metal cover. The built-in heat pipe has an evaporating portion, a condensing portion, and a bent portion between the evaporating portion and the condensing portion. Thermal interface materials are applied in the cavity such that the evaporating portion is thermally coupled to the metal vessel, and the condensing portion is thermally coupled to the metal cover. The metal vessel has a flat surface opposing to the cavity for attaching to a heating surface of a semiconductor package.