A locking device used for locking a heat sink to an electronic device, includes two clips each having a clamping portion. The clamping portion of each has a first pressing portion, a second pressing portion spaced from and in alignment with the first pressing portion, and a catch depending between the first and second pressing portions. An operating portion extends from the second pressing portion of the clamping portion. The retention module includes a bottom with an opening defined therein, and two pairs of first and second clasps extending from the bottom for locking the two clips. When the retention module and the clips are in a locked position, the catches of the clips engage with the first clasps of the retention module, the operating portions engage with the second clasps of the retention module.