Pulse reaction, TPO/TPD (temperature programmed oxidation/temperature programmed desorption), XPS (X-ray photoelectron spectroscopy) and DRIFT (diffuse reflectance infrared transform spectroscopy) have been used to investigate the role of oxygen on SCR (selective catalytic reduction) of NO over Cu/ZSM-5 zeolites, especially during the early stage. Pulse reaction shows that propene is deposited on Cu ions and inhibits the adsorption of NO on them. It is necessary for oxygen to make Cu sites clean by deep oxidation of propene. In the presence of O-2, SCR at 623 K is accompanied by coke deposition on the zeolite. This coke can be discriminated from carbonaceous species formed on Cu in the absence of O-2 from the result of TPO. XPS results implies that Cu(II)-O species play a crucial role in forming Cu(II)-NO2 species, and subsequently are well correlated with the activity of NO SCR.