A mounting device (10) for mounting a heat sink (20) to a circuit board (40), includes a locking member (16) extending through the heat sink and including a through hole (162) and barbs (172) formed in one end portion thereof for engaging with the circuit board, an operation member (12) including a cylinder (122) and a rod (128) extending from the cylinder into the through hole of the locking member. First and second bumps (178, 179) are formed in the locking member, and a pair of slots (126) is defined in the cylinder for engaging with the first and second bumps at first and second positions respectively. A spring (14) is received in the cylinder, surrounding the rod and compressed between an end of the cylinder and the first bumps. The rod expands the barbs outwardly at the second position and releases the barbs at the first position.