The Ce-Cu-SAPO-18 samples were prepared by the ion exchange method. Physicochemical properties of the samples were systematically characterized by a number of analytical techniques, and Ce doping and hydrothermal temperature effects on NH.sub.3-SCR activity of Cu-SAPO-18 were also investigated. The results show that doping of Ce increased NH.sub.3-SCR activity and hydrothermal stability of the Cu-SAPO-18 sample. After Ce doping, a more amount of the isolated Cu.sup.2+ ions entered the D6R and the catalyst structure was more stable. The structure and catalytic activity of Ce-Cu-SAPO-18 remained almost intact after hydrothermal aging at 650 °C. After hydrothermal aging at 850 °C, however, the structure of Ce-Cu-SAPO-18 was greatly destroyed, and its catalytic activity declined remarkably. The high-temperature hydrothermal aging treatment led to decreases in amount of the isolated Cu.sup.2+ ions and acidic sites, destroying in the zeolitic structure, and drop in NH.sub.3-SCR activity.