A heat sink has a base (10), a plurality of fins (12, 14) and a central receiving groove (16) in the fins. A first positioning portion (1250) extends from a first fin at a side of the receiving groove and a positioning slot (1256) is defined therein. The clip includes a main body (23) and a pair of legs (25). The main body includes a retaining portion (230) and a first pressing portion (232). The main body is received in the receiving groove and the first pressing portion is accommodated in the positioning slot. The retaining portion is offset from the first pressing portion. The legs can be pressed to securely connect with a printed circuit board (3), whereby the heat sink can have an intimate contact with a CPU (4).