We describe a simple method to control the size of Pt nanoparticles with the use of an alcohol reduction. They then are assembled on the electrode by an electrophoretic deposition. The mean diameter of monodispersed Pt nanoparticles can be controlled from 19 to 33 Angstrom in one-step reaction by changing the kind and/or the concentration of alcohol in water and the amount of protective polymer, poly(N-vinyl-2-pyrrolidone) (PVP). The monodispersed Pt nanoparticles of smaller diameter are obtained in the order of methanol > ethanol > l-propanol, indicating that a faster reduction rate of [PtCl6](2)- ions is an important factor to produce the smaller particles. The particle diameter decreases linearly with concentration of alcohol in water. Furthermore, increasing the amount of PVP makes the size of Pt nanoparticles smaller, the size distribution remaining quite narrow. By the combination of the one-step reaction with the stepwise growth reaction, Pt nanoparticles of arbitrary diameter in the range 19-50 Angstrom can be obtained. The Pt nanoparticles obtained here have fee structures such as bulk Pt. By using an electrophoretic deposition, more Pt nanoparticles move toward the anode with the increase of both the applied voltage and the electrophoretic time to form the Pt nanoparticle monolayer. Using the high concentration of Pt dispersion is effective to form the Pt nanoparticle monolayer.