N2O decomposition on Co-MgO was studied under high (6.67 kPa) and low (75 Pa) N2O pressure and the effect of reductant (C2H6, NH3) was studied. The activity decays because of the strong adsorption of oxygen produced, while the reductant removed the adsorbed oxygen giving a steady catalysis. The reaction between NH3 and excess amount of O-2 produced N2O as a main product. Although N2O gives the same intermediate (O) as O-2 does, the former decomposition seems to proceed faster than the latter on Co-MgO. The reaction mechanism studied here was compared with the SCR (NO-NH3 in O-2) reaction on V2O5-TiO2. Since NH3, N2O and O-2 gives only N-2 and water, Co-MgO can be a possible catalyst used in the boiler exhaust to reduce N2O concentration by adding ammonia.