Nowadays, preparation of special and e ective nanocatalysts is a hot research topic. Obviously, the size and structure of the active site of a catalyst plays a key role in catalytic activity. In this novel research, the Pt nanocatalyst supported on -Al2O3 was prepared in two di erent microemulsion systems to clarify the e ect of microemulsion essence on catalyst character and activity. Two types of microemulsion systems were constructed from Triton X-100/2-butanol as surfactant and co-surfactant, i.e., n-heptane as organic phase and H2O + H2PtCl6 (or HCl(aq) + H2PtCl6) as aqueous phase, to investigate the e ect of HCl on the shape and structure of the prepared nanocatalysts. Shape, size, and size distribution of the Pt nanoparticles on the alumina were monitored by cyclic voltammetry and SEM analysis. Activity of the nanocatalysts was determined by reduction of p-nitrophenol as a reaction model.