A novel Co-Ce-Ti composite oxide catalyst prepared by the hydrothermal method has been developed for the selective catalytic reduction of NOx by NH3. Co-Ce-Ti catalyst exhibited high NH3-SCR performance in a wide temperature window, which is superior to that of Co-Ti and Ce-Ti catalysts. Catalyst characterization results revealed that over Co-Ce-Ti catalyst the redox cycle (Co3+ + Ce3+<-> Co2+ + Ce4+) plays an important role for the high catalytic NH3-SCR performance. The synergetic effect between Co and Ce over Co-Ce-Ti catalyst resulted in both improved reducibility and increased acidity. In situ DRIFTS results clarified that over Co-Ce-Ti catalyst more reactive NH3/NH4+ species is formed, thus promoting the NH3-SCR of NOx to proceed. The present Co-Ce-Ti catalyst has great potential to be used for practical deNOx application.