A power adapter includes an insulating casing including an enclosing wall and defining a sealing chamber therein, a power transforming unit disposed in the sealing chamber, and a heat sink device including a heat dispersing member supported within the sealing chamber at a position between the enclosing wall and the power transforming unit to thermal-communicate with the power transforming unit, wherein the heat dispersing member has an air insulating cavity formed above the power transforming unit in such a manner that when the power transforming unit generates heat for heating up air within the air insulating cavity, the heat is diffused throughout the air insulating cavity so as to evenly disperse on the enclosing wall of the insulating casing.