A cooling device includes a boiling chamber containing working fluid therein. The chamber includes a bottom wall for contacting with a heat generating component and a cover hermetically connected with the bottom wall. The cover includes a top wall and a bulge formed at a periphery thereof. The bulge includes a pair of spaced side walls, one of the side walls connecting with the top wall and the other of the side walls connecting with the bottom wall. A buffering region is formed between the side walls, which is capable of absorbing agitation wave generated by boiling fluid to thereby allow the boiling fluid to be capabling of continuously and stably contacting with and exchanging heat from the top wall of the boiling chamber when the fluid boils. A heat sink is arranged on the cover of the boiling chamber for transferring heat from the boiling chamber to environment.