A fastening device for a hard disk drive (HDD) includes two fastening members and a bottom plate. Each fastening member includes a sidewall, and a top wall and a bottom wall perpendicularly extending from opposite sides of the sidewall. The top walls are fastened to a bottom of the HDD. An engaging hole is defined in each bottom wall. The engaging hole includes a first hole and a second hole. A positioning tab extends from one of the fastening members. A block is formed on the positioning tab. Two protrusions protrude from the bottom plate. A positioning portion is formed on the bottom plate. The HDD is operable to slide to allow the protrusions to engage in the first holes. The positioning tab extends through a through hole bounded by the positioning portion and bottom plate, to allow the block to be blocked by the positioning portion.