A retaining mechanism, used for retaining an electronic component including a support bracket having an end part formed with an engaging hole, includes a mounting block having a mounting hole for receiving the end part of the support bracket, and a through hole communicating with the mounting hole. A seat body covers the mounting block, and has a bolt hole communicating with and disposed above the through hole, and a limiting slot communicating with one side of the bolt hole. A locking bolt includes a shank disposed slidably in the bolt hole, and a stopper block connected transversely to the bottom portion and slidable upward and downward along the limiting slot. The shank has a bottom portion adapted to engage releasably the engaging hole when the through hole is aligned with the engaging hole.