We report on the synthesis of reduced graphene oxide (RGO) supported Pt nanoparticles (NPs), in which was achieved via one-pot ultrasound-assisted polyol synthesis (UPS) reactions, Through the UPS mechanism, Pt(acac)₂ and graphene oxide (GO) were reduced simultaneously as Pt NPs and RGO, respectively. Based on observations from a series of control groups, we would like to propose a mechanism of the NPs' formation so called as seeding that enables control of the amount of reduced graphene oxide. Characterization of the composites by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermal analysis (DTA) revealed reduction steps that platinum is first and graphene oxide is second. The Raman spectra of the Pt/RGO composites showed that a well reduction was happened during UPS. The merits of UPS simultaneous reduction in developing reduced graphene oxide supported composites are discussed.