Pt metallic nanoparticles have been synthesized by the polyol method and stabilized with poly(vinylpyrrolidone) (PVP) at higher temperatures (130°C). The structure of the particles has been determined using transmission electron microscopy and high-angle annular dark field (HAADF). The combination of these experimental techniques allowed the characterization of the structure and the determination of the distribution of the platinum nanoparticles. The electrochemical behavior of synthesized platinum nanoparticles (2-4 nm) was investigated toward oxygen reduction reaction by cyclic voltammetry in 0.5 M H2SO4. Experiments were carried out through synthesized solution and performed using a standard three-electrode cell configuration employing a gold wire counter electrode and an H+/H2 reference electrode. The synthesized electrocatalyst, in comparison with polycrystalline Pt, showed a variation of the catalytic activity towards oxygen reduction.