A low-temperature ammonia synthesis process is required for on-site synthesis. Here, we report that barium-doped calcium amide (Ba-Ca(NH2)2) significantly enhances the ammonia synthesis activities of Ru and Co by two-orders of magnitude more than that of a conventional Ru catalyst below 300ºC. Furthermore, the present catalysts are superior to that of the wüstite-based Fe catalyst known as a highly active industrial catalyst at low temperatures and pressures. Nanosized Ru-Ba core-shell structures are self-organized on the support during hydrogen pretreatment, and the support material is simultaneously converted into a mesoporous structure with a high surface area (>100 m2 g-1). These unique self-organized nanostructures account for the high catalytic performance in low-temperature ammonia synthesis.