Graphene-supported Pt nanoparticles were prepared by a self-regulated reduction method without using any extra reductive agent. Unassisted reduction of Pt ions by the oxygen-containing functional groups on graphene was demonstrated. X-ray diffraction (XRD) showed a (200) peak of face-centered cubic Pt crystals and energy dispersive X-ray spectroscopy (EDS) further confirmed the presence of Pt. Transmission electron microscopy (TEM) depicted good dispersion of the Pt nanoparticles on graphene. The particle sizes estimated by TEM and XRD ranged from 2 to 6 nm. In comparison, the Pt nanoparticles reduced using ethylene glycol as an extra reducing agent exhibited larger sizes, a wider spread of size distribution, and less uniform dispersion on graphene. The electrocatalytic activity of Pt on graphene was verified by cyclic voltammetry. In addition, Raman scattering spectroscopy showed an increase in D- to G-peak ratio and an effect of surface-enhanced Raman scattering for the graphene decorated with Pt nanoparticles.