Efficient and stable catalysts for ammonia synthesis under mild conditions are required to meet the strong demand for NH3 as an important precursor chemical and hydrogen carrier. Here we report that during ammonia synthesis, flat-shaped Ru nanoparticles with a narrow distribution (2.1 ± 1.0 nm) and self-organized on Ca­(NH2)2 exhibit high catalytic performance far exceeding alkali-promoted Ru-based catalysts in yield and turnover frequency (TOF). This catalyst enables continuous NH3 production, even at 473 K under ambient pressure. During ammonia synthesis, Ru nanoparticles are distinctly anchored on the surface of Ca­(NH2)2 by strong Ru–N interaction, which leads to the epitaxial growth of Ru on the support surface. The high catalytic performance is due to the formation of high-density flat-shaped Ru nanoparticles and high electron donor ability at the Ru/Ca­(NH2)2 interface. The catalytic stability is significantly improved by Ba-doping of Ca­(NH2)2, and no degradation was observed after ca. 700 h of operation.