A liquid cooled heat sink includes: a casing defining an inner space and provided with a partitioning wall dividing the inner space into upper and lower chambers and formed with a fluid passage in fluid communication with the upper and lower chambers, the casing being formed with a fluid inlet in fluid communication with the upper chamber and adapted to be connected to an external cooling device; a fin unit provided in the lower chamber; and a pump mounted on the casing and having a suction end disposed in the upper chamber, extending through the partitioning wall, and in fluid communication with the lower chamber, and a discharging end disposed in the upper chamber and adapted to be connected to an external cooling device.