A mounting structure for an in-wheel motor system is provided. The mounting structure includes the in-wheel motor system installed in a wheel of the vehicle, and a torsion beam axle (TBA) assembly to fix the in-wheel motor system to a vehicle body, wherein a rear portion of a motor housing includes a step portion formed in a stepped manner to have a decreased diameter, and a ring-shaped fastening face vertically arranged from the step portion, and the TBA assembly includes a trailing arm provided with a through hole to surround the step portion of the motor housing, a mount integrated with the trailing arm to fix the trailing arm to a vehicle body, and a plurality of fastening bolts to fix the motor housing to the trailing arm, wherein, when the step portion is fitted and coupled into the through hole, the fastening face closely contacts the trailing arm.