Experiments were conducted to study the effects of pretreating Cu-Y zeolite catalysts on the reduction of nitric oxide with ammonia. Changing the oxidation state and environment of the copper in Cu-Y changed the optimal reaction temperature of the NO-NH3 reaction and the activity of this catalyst. At the optimal temperature, the activity of Cu-Y after dehydration was higher than that of hydrated Cu-Y. In addition, at the optimal temperature, the activity of Cu-Y after the NO-NH3 reaction followed by oxidation or ammonia pretreatment followed by oxidation was higher than that of dehydrated Cu-Y. The activity of Cu-Y at the optimal temperature was decreased by reduction at higher temperature but increased by reduction at lower temperature. Furthermore, the activity of Cu-Y increased as the copper loading was increased. The optimal reaction temperatures of the NO-NH3 reaction over various pretreated Cu-Y catalysts were 106, 126, 220 and 300 degrees C, respectively.