In this paper, we describe the synthesis and growth mechanism of highly monodispersed platinum nanocubes. The platinum nanocubes are synthesized by the decomposition of a platinum precursor in a hydrogen atmosphere. The morphology and size distribution of the platinum particles formed has been studied with HRTEM. By controlling the concentration of the platinum precursor, we demonstrate that at low concentration, it is possible to grow polydispersed nanocubes with {1,0,0} facets. Increasing the concentration of the precursor changes the growth mechanism, resulting in the formation of highly monodispersed platinum nanocubes. Highly monodispersed platinum nanocubes are formed in a two-step growth mechanism with initial growth of the {1,1,1} facets followed by secondary growth filling the {1,0,0} facets. The particle monodispersity facilitates the formation of long-range arrays of nanocubes.