A liquid cooling device includes a heat-dissipating unit (1) and a heat-absorbing unit (2). The heat-dissipating unit (1) connects with the heat-absorbing unit (2) by an inlet pipe (4) and an outlet pipe (3) to form a closed circulation loop. The heat-dissipating unit (1) includes a base (10), a tank (20), a heat-dissipating body (30) beside the tank (20) and a cover (40), which are assembled together as a single unit. A pump (23) is placed in the tank (20) and operated to pump liquid coolant to flow in the circulation loop. During flowing through the base (10) and the cover (40), the liquid coolant transfers heat to the heat-dissipating body (30).