Partial NO is oxidized to NO2 during the emission gas through diesel oxidation catalysts (DOC). Proper NO2 /NOx ratio increases the low-temperature soot oxidation rate of diesel particulate filter (DPF) and the NOx conversion efficiency of selective catalytic reduction (SCR). In order to study the degreening effect of DOC, in-situ diffuse reflectance infrared Fourier transform spectroscopy (In-situ DRFTS), H2 temperature-programmed reduction (H2-TPR) and High-revolution transmission electron microscope (HR-TEM) were performed to characterize the Pt/Al₂O₃ catalyst. A larger amount of nitrate and nitrite species was observed in the fresh Pt/Al₂O₃ catalysts blow 300 oC. It is shown that PtOx species that interacted strongly with support are the main cause of degreening effect. The particle size of Pt has no significant change after 3-cycles temperature-programmed reaction.