Several K-promoted Ru/C catalysts with different K precursors were prepared. The heat treatment at different temperatures (400–500 °C) was carried out to investigate the effect of K promoters on the stability of carbon-supported Ru catalysts. The catalysts were characterized by XRD, TEM, CO-TPR, temperature-programmed heat treatment study in argon, H₂-TPR and H₂ chemisorption. The results revealed that the sample with KNO₃ showed the highest heat resistance. The presence of K promoters increased the number of hydroxyl groups, and potassium hydrogen carbonate was found in the reduced catalysts obtained from KOH or K₂CO₃. These results led to an increase in methane formed at low temperature, which did not significantly affect the catalytic activities of K-promoted Ru catalysts. The formation of CO was unavoidable for all Ru catalysts at high temperature, which may destroy the structures of carbon supports. The ammonia synthesis activity decreased because more hydrogen may participate in the formation of CO and CO₂.