To provide an upper end mounting part structure of a rear strut assembly capable of improving the NVH performance of a vehicle by forming an upper end side mounting part, to which a load of the rear strut assembly is transmitted in the space of a box structure to increase stiffness, and accordingly reducing travel noise and idle noise.

In the upper end mounting part structure, the internal surface of a quarter inner panel, a rear wheel housing inner panel, a guide hole, and a bolt hole are formed. The structure is composed of a wheel housing cover set up by being welded to the internal surface of the rear wheel housing inner panel and forming the space of the box structure between itself and the rear wheel housing inner panel, a reinforcing bracket welded to the wheel housing cover connected to the bolt hole, and a welding nut.