Cubic assembly composed of platinum nanoparticles has been prepared by a simple reaction between H2PtCl6•6H2O, NaOH and NaBH4 in the presence of deionized gelatin. Gelatin played a decisive role as an inhibitor of the direct reaction of NaBH4 with H2PtCl6•6H2O and coagulation of the growing platinum in producing the cubic assembly composed. In the absence of gelatin, we can only obtain irregular large platinum particles. With decreasing the temperature, the viscosity of the solution increased. The diffusions of BH4- ions and PtCl62− ions are slower. The assembly composed of platinum nanoparticles was synthesized at 10 oC. The products were characterized by X-ray powder diffraction, scanning electron microscope, X-ray photoelectron spectra and UV-visible absorption spectroscopy. The scanning electron microscope images show the as prepared platinum with cubes and the average dimensions is about 0.35-0.45 μm. The UV-visible absorption spectrum of the as prepared platinum particles dispersed in ethanol solution shows one absorption peaks at 215nm. The size of the subunits of the polycrystalline platinum particles was calculated to be about 4 nm according to half width of the diffraction peaks using Debye–Scherer equation.