Addition of cobalt oxide (Co₃O₄) as a heating promoter, absorbing microwave energy with high efficiency, was suggested in order to accelerate microwave heating of zeolite adsorbent or catalyst. The effect of the addition of Co₃O₄ on the microwave heating of Co-modified mordenites prepared by the impregnation method and physical mixing was investigated. Catalytic oxidation of ethylene on Co-modified mordenites was also examined under microwave irradiation. Co₃O₄ promoted microwave heating of the mordenite bed when Co₃O₄ and mordenite were mixed physically. The physical mixture with Co₃O₄ of 50 wt% was heated up to 518 K by microwave irradiation. The desorption peak of water for pellet type Co-modified mordenite was very sharp in microwave heating after pre-adsorption of water vapor and most of adsorbed water desorbed in the initial stage of the microwave heating. The sharp desorption peak of water indicates that the pelletized sample was heated to higher temperatures than powder one. In the case of the pelletized Co-modified mordenite with Co₃O₄ of 75 wt%, adsorbed water on mordenite was almost desorbed during microwave irradiation. Co₃O₄ had both functions of the heating promoter and catalyst, and ethylene was removed with high conversion (93%) when the pelletized mixture of Co₃O₄ (75 wt%) and mordenite was irradiated by microwave under steady feed of ethylene, water vapor, and oxygen.