A computer enclosure (1) includes a chassis (50), a rear window frame (70), an expansion card seat (10) and a retaining plate (80). The chassis includes a bottom plate (53), a rear plate (52) and a side plate (53). The rear plate forms a slideway (55) and a beam (59) at a top thereof, and defines an opening between the slideway and the beam. The rear window frame includes a bottom panel (71) having a slide board (72), and an expansion slot frame (76) received in the opening. The seat includes a base plate (12) received in the opening, a supporting plate (16) extending from the base plate, a locking plate (34) extending from the supporting plate and engaging with the beam. The retaining plate is detachably attached to the seat for retaining expansion cards to the expansion card slots in the chassis.