Citric acid (CA) was used to modify graphited activated carbon (AC) for improving size distribution of Ru particles and catalytic activity of the Ru/AC catalyst. The influence of CA on the texture of AC and Ru/AC, Ru distribution and catalytic activity were investigated. TEM, TGA, TPD-MS, CO pulse chemisorption and N-2 physi-sorption indicate that CA modified AC creates more functional groups and thus more homogeneous dispersion of Ru nanoparticles. TEM images show most of the Ru particles were ca. 2-2.5 nm for the CA4 + Ru4/AC catalyst. The increase of ammonia synthesis activity for Ru-Ba-K/AC catalyst is more than 16% when the content of CA in AC is 4 wt.% at 673 K and 10,000 h(-1).