A mounting device for hard disk drive includes a cage and a frame. The cage defines a receiving space. The receiving space includes a bottom plate. The frame includes a mounting plate and two limiting plates. The hard disk drive is secured between the two limiting plates. A gap is defined between the mounting plate and the bottom plate. A positioning protrusion is located on the bottom plate. A first mounting piece and a second mounting piece extend from the mounting plate. Two positioning pieces extend from the first mounting piece. The second mounting piece abuts the positioning protrusion, the first mounting piece and the second mounting piece are secured to the positioning protrusion, and the two positioning pieces abut the bottom plate, to prevent the second mounting piece from jolting towards the bottom plate.