The direct decomposition of NO proceeds catalytically on alkaline earth metal oxides supported on Co3O4. Activity decreased in the order of Ba/Co3O4 > Sr/Co3O4 > Ca/Co3O4 >> Mg/Co3O4 approximate to Co3O4. The catalytic activity of supported alkaline earth metal oxides was found to be directly related to not only the number of surface basic sites, which corresponds to the surface density of alkaline earth elements, but also to the strength of basicity, suggesting that NO decomposition proceeds on alkaline earth metal oxides.