A liquid cooling device includes a casing having an outer wall, a bottom base attached to a bottom end of the outer wall and a top cover attached to a top end of the outer wall. The casing defines a receiving space therein for receiving an impeller therein. A sealed chamber is defined in the casing and isolated from the receiving space. A motor driving unit is received in the sealed chamber and magnetically interacts with the impeller to drive the impeller to rotate. The motor driving unit comprises a motor having a stator and a printed circuit board electrically connecting with the stator. The top cover covers the motor driving unit and thermally contacts with electronic components on the printed circuit board for dissipating heat generated by the electronic components when the motor driving unit is operated.