Abstract: Different size platinum nanoparticles, ranging in size from 1.8-14.1 nm, were prepared by multi-step reduction of H2PtCl6 by hydrogen adsorbed on platinum atoms. Transmission electronic microscopy and X-ray diffraction were used to characterize the nanoparticles. After thirty-two iterations of the reaction, the diameter of platinum nanoparticles increased from 1.8 nm to 14.1 nm. The average iterative increase was approximately 0.4 nm. The size distribution of the nanoparticles was narrow. Synthesis procedures which allow for control of platinum nanoparticle size offer the possibility for the further research into how the sizes of nano-catalysts effect catalytic activity.