A computer enclosure includes a cage (10) with a rear panel (14), a side panel (20) with a flange (122a) and a fastening device (30). The fastening device includes a holder (40) attached to the side panel, an slider (50), an operator (60) with protruding bars (66), and a spring (70) disposed between the slider and the holder. The holder includes a base (41) rotatably receiving the operator and pairs of hooks (447) for movably holding the slider. The slider includes a pair of cutouts (522) and a fastening portion (55). The operator is rotated and the protruding bars engage with the cutouts to move the slider forwardly and compress the spring. The side panel is attached to the cage with the flange abutting against the rear panel. The protruding bars move the slider backward and the fastening portion abut against inner surfaces of the flange, with the operator released.