An expansion card mounting apparatus includes an enclosure, a slot cover of an expansion card, and a retaining member. The enclosure includes a side wall which defines an expansion card slot. A support plate is formed on the side wall and located above the slot. The slot cover includes a fixing portion. A recess is defined in the fixing portion. The slot cover shields the expansion card slot with the fixing portion placed on the support plate. The retaining member is pivotably mounted on the side wall. The retaining member includes a pressing plate and a boss formed on the pressing plate. The pressing plate presses the fixing portion on the support plate, and the boss located in the recess.