A heat dissipation device includes a heat sink having a heat conducting core and a plurality of fins extending from the core. The core has a downward extension portion extending away the fins for contacting a heat generating device. A locking device includes a locking plate defining a central opening accommodating the extension portion of the core of the heat sink therein and has a plurality of locking arms extending from the locking plate. Pluralities of cutouts circumferentially surrounding and communicating with the opening are defined in the locking plate. The extension portion of the core of the heat sink is punched to press the locking plate of the locking device toward the heat sink and have portions thereof engaged in the cutouts of the locking plate of the locking device, whereby the locking device is stably fixed to the heat sink.