A heatsink mounting unit prevents a heatsink cooling an electronic device mounted on a printed circuit board from a breakaway. The heatsink mounting unit includes a flat spring having a first end part formed with a combining arm combined to the printed circuit board by a fastening device and a second end part having a hook. The flat spring elastically presses the heatsink against the printed circuit board. The heatsink mounting unit also includes a hook holding member combined to the printed circuit board and having a hook hole to which the hook is hooked. Thus, the present invention provides a heatsink mounting unit which is easily mounted and effectively prevents a heatsink from a breakaway.