A fastening device for a hard disk drive (HDD) includes a bottom plate and two fastening members. Two protrusions protrude up from the bottom plate. A positioning portion is formed on the bottom plate. The positioning portion defines a hooking slot. Each fastening member includes a connection wall, and a top wall and a bottom wall perpendicularly extending from top and bottom sides of the connection wall. The top walls are fastened to a bottom of the HDD. Each bottom wall defines an engaging hole. The engaging hole includes a large first hole and two small second holes communicating with opposite sides of the first hole. A latch is formed on one of the fastening members. The HDD is operable to slide to allow the protrusions to enter the corresponding second holes from the corresponding first holes, and to allow the latch to engage in the hooking slot.