A liquid cooling device (10) includes a heat sink (12), a reservoir (14) distant from the heat sink, a duct (16) connecting with the reservoir, a flexible tube (18) connected to the duct and the heat sink, and a heat-transfer member (19) in the liquid cooling device. The liquid cooling device is filled with liquid therein. The heat-transfer member is a heat pipe including an evaporating segment (192) fixed to the heat sink and a condensing segment (194) extending freely in the reservoir.