The effects of promoters K, Ba, Sm on the chemisorption and desorption of hydrogen and nitrogen,dispersion of metallic Ru and catalytic activity of active carbon (AC) supported ruthenium catalyst for ammoniasynthesis have been studied by means of pulse chromatography, temperature-programmed desorption, and activitytest. Promoters K, Ba and Sm increased the activity of Ru/AC catalysts for ammonia synthesis significantly, andparticularly, potassium exhibited the best promotion on the activity because of the strong electronic donation tometallic Ru. Much higher activity can be obtained for Ru/AC catalyst with binary or triple promoters. The activityof Ru/AC catalyst is dependent on the adsorption of hydrogen and nitrogen. The high activity of catalyst could beascribed to strong dissociation of nitrogen on the catalyst surface. Strong adsorption of hydrogen would inhibit theadsorption of nitrogen, resulted in decrease of the catalytic activity. Ru/AC catalyst promoted by Sm2O3 showsthe best dispersion of metallic Ru, since the partly reduced SmOx on the surface modifies the morphology of activesites and favors the dispersion of metallic Ru. The activity of Ru/AC catalysts is in accordance to the correspondingamount of nitrogen chemisorption and the desorption activation energy of nitrogen. The desorption activation energyfor nitrogen decreases in the order of Ru＞Ru-Ba＞Ru-Sm＞Ru-Ba-Sm＞Ru-K＞Ru-K-Sm＞Ru-K-Ba＞Ru-K-Ba-Sm,just opposite to the order of catalytic activity, suggesting that the ammonia synthesis over Ru-based catalyst iscontrolled by the step of dissociation of nitrogen.