The present invention is to provide a slot assembly of hard disk tower, wherein the slot assembly disposed at one end of the hard disk tower comprises an upper slot assembly cover at the top of the hard disk tower having a plurality of upper directing slots; a lower slot assembly cover at the bottom of the hard disk tower having a plurality of lower directing slots, and a circuit board disposed between said upper and lower slot assembly covers. Each lower directing slot which corresponds to one of the upper directing slots is capable of holding a hard disk tray, such that the space within the hard disk tower will be fully utilized and is capable of holding more hard disk trays because of closely disposing the slot assembly covers at the top and bottom of the hard disk tower.