A mounting apparatus includes a supporting bracket, a locking member, and a holding mechanism. The supporting bracket is fixed to a slide rail and includes two inserting pins extending through the two corresponding through holes of a rack post. The locking member is slidably mounted to the supporting bracket. When the locking member slides to abut against a front surface of the rack post, the slide rail is prevented from moving backwards to make the inserting pins disengage from the through holes. When the locking member slides to space away from the rack post, the slide rail is enable to move backwards to make the inserting pins disengage from the through holes. The holding mechanism abuts against the locking member to prevent the locking member from sliding. The holding mechanism is operable to move away from the locking member.