An electronic device includes a casing, a fan, and a heat sink. The casing defines a plurality of through holes therein. The fan defines air outlet at one side thereof facing the through holes of the casing. The air outlet includes a first portion and a second portion. Air pressure in the first portion is larger than air pressure of the second portion. The heat sink includes a first fin set arranged on the first portion and a second fin set arranged on the second portion. A first passage is defined between each two neighboring first fins. A second passage is defined between each two neighboring second fins. A width of the second passage is less than that of the first passage.