A drop-out mounting structure of a bicycle frame to inhibit a drop-out member from being separated from the bicycle frame is disclosed. The drop-out mounting structure of the bicycle frame for mounting a drop-out member as a wheel mounting unit to a drop-out mounting unit of a bicycle frame through a bolt may include an assembly slot formed at the drop-out mounting unit to insert the drop-out member inside the drop-out mounting unit and a stopper unit formed at an upper end part of the drop-out member with a length that is longer than an opening end of the assembly slot.