A fastening device for expansion cards that is capable of connecting to a slot on a normal motherboard and fastening an expansion card in the slot is disclosed. The fastening device includes a top pressing unit and two side fastening units, wherein the two side fastening units are connected to two ends of the top pressing unit, respectively, and each side fastening unit is provided with an opening. When two clip units disposed on the two ends of the slot pass through the openings of the two side fixing units respectively, the top pressing unit presses the top edge of the expansion card inserted into the slot and the two side fastening units respectively press and fasten the two clip units so as to restrain the clip units from unlocking and to prevent the expansion card from becoming loose in the slot.