A fastening device for a hard disk drive (HDD) includes a bottom plate and two fastening members. Two protrusions protrude from the bottom plate. A positioning portion is fastened to the bottom plate. The positioning portion includes an operation tab. A block protrudes from the operation tab. Each fastening member includes a sidewall, and a top wall and a bottom wall extending from top and bottom sides of the sidewall. The top walls are fastened to a bottom of the HDD. Each bottom wall defines an engaging hole. The engaging hole includes a first hole and two second holes communicating with the first hole. When the HDD is being fastened, the HDD is slid to allow each protrusion to enter one of the second holes, and to allow the block to block a corresponding side of the HDD.