A heatsink assembly includes a heatsink which has a base board and fins extending from a top thereof. The heatsink is directly put on the chip set. A positioning device includes a rectangular frame which is mounted to the heatsink and includes two first extensions and two second extensions extending from two pairs of opposite sides thereof. Each first extension has a hook extending from an inside thereof so as to hook the circuit board and the second extensions each have a first inclined surface engaged with the inclined surface defined in a periphery of the chip. Two flexible rods extend from the two opposite sides of the rectangular frame and each flexible rod has a pressing portion which presses on the top of the base board of the heatsink.