Pt/Al₂O₃ catalysts were prepared by an excess impregnation method with Pt(NH₃)₂(NO₂)₂ as the precursor. Then they were calcined in four different atmospheres (H₂, O₂, NO, NH₃), and characterized by N₂ adsorption and desorption test, X-ray diffraction, H₂-temperature programmed reduction (TPR) test, CO pulse adsorption test, and CO in-situ diffuse reflectance infrared Fourier transform spectroscopy (CO in situ DRIFTS). Results show that Pt/Al₂O₃ catalyst calcined in 1vol% H₂/N₂ atmosphere exhibits the best catalytic oxidation performance for CO and C₃H₆, because of the generation of numerous small-sized and highly dispersed Pt nanoparticles caused by the reducing calcination atmosphere.