A heat sink allowing a reduced thickness of an electronic device includes a housing, a wick structure, and a heat transfer fluid. The housing, comprising an upper housing and a lower housing, are opposite and form a sealed cavity. The wick structure and the heat transfer fluid are received in the cavity, the liquid absorbing heat and becoming gaseous upon reaching a certain temperature. The area of the upper housing is larger than the area of the lower housing. The upper housing forms a protrusion with respect to an edge of the lower housing. An electronic device using the heat sink is also provided.