A locking device (10) for securing a heat sink to an electronic device includes a rectangular main frame (20), four first fasteners (30) respectively pivotably attached to four corners of the main frame and two second fasteners (40) pivotably attached to the main frame at opposite sides thereof. Each of the first and second fasteners respectively is capable of rotating relative to the main frame from a non-stretched position to a stretched position for attachment of the heat sink to the electronic device.