The Ru/CeO2 (JRC-CEO-3) catalyst prepared by NaBH4 reduction showed a high activity for NH3 decomposition compared with the other catalysts prepared in this work; however, this catalyst gradually lost its activity due to the accumulation of adsorbed species produced during the reaction. Cs significantly improved the catalytic performance of the Ru/CeO2 catalyst, and the optimum Cs/Ru molar ratio was 0.4 because excess Cs species gradually covered the exposed Ru metal active sites. Cs-Ru/CeO2-0.4 catalyzed the NH3 decomposition reaction without H-2 pretreatment because the catalyst was activated by NH3 or H-2 produced by NH3 decomposition. Moreover, Cs not only increased the N-H bond dissociation rate but also decreased the effect of the H-2 partial pressure on the catalytic performance. Consequently, the Cs-Ru/CeO2-0.4 catalyst exhibited a high, stable activity (NH3 conversion: 92% at 623 K and 60% at 573 K) with a gas hourly space velocity of 2000 mL-NH3 g-cat(-1) h(-1).