A computer enclosure includes a sidewall and a fan mounting apparatus. The sidewall defines a ventilation area and four slide slots around the ventilation area. The fan mounting apparatus includes two opposite first position member, and two opposite second position members alternately arranged with the first position members. The first and second position members are slidably connected end to end and cooperatively define a ring. The first and second position plates are moved toward or away from each other. The first and second position members are slidably and respectively connected to the slide slots, to decrease or increase a diameter of the ring for selectively mounting different sized fans.