A liquid cooling device (1) includes a base (2) enclosing working fluid therein and absorbing heat from a heat-generating electronic component, a heat sink (10) fluidically connecting with the base, and a pump (3) driving the working fluid to circulate between the base and the heat sink. The heat sink has closed top and bottom ends, and includes an outer tube (100) and an inner tube (30) disposed in the outer tube. First and second receiving chambers (34, 14) are respectively formed in the inner tube and between an inner surface of the outer tube and an outer surface of the inner tube for receiving the working fluid therein. The first receiving chamber communicates with the second receiving chamber at an end thereof so that the working fluid flows from the first receiving chamber to the second receiving chamber.