The reductive removal of dissolved oxygen (DO) in water by hydrazine was studied on Mn, Fe, Co and Ni oxide catalysts supported on activated carbon fiber (ACF). The Co(0.36)/ACF catalyst showed the best activity among the prepared catalysts, even better than the noble metal catalyst. The reaction orders of DO and hydrazine on the Co(0.36)/ACF catalyst were obtained as 0.36 and 0.50, respectively, and the activation energy as 21.5 kJ mol(-1). The catalytic reaction of DO reduction on cobalt oxide was carried out through the cycle composed of the formation of oxygen vacancy by hydrazine and the occupation of oxygen vacancy by DO, based on electrochemical measurements and kinetics. The exceptional activity of cobalt oxide in the reduction reaction of DO by hydrazine was attributed to the involvement of lattice oxygen to activate DO efficiently.