Ammonia decomposition over nitrided MoNx/alpha-Al2O3 and NiMoNy/alpha-Al2O3 catalysts was investigated at temperatures 600-750 degrees C, GHSV = 1800-3600 h(-1). The conversions of ammonia decomposition over nitrided MoNx/alpha-Al2O3 and NiMoNy/alpha-Al2O3 catalysts at 650 degrees C are as high as 98.7% and 99.8%, respectively. The catalytic activity of ammonia decomposition varies significantly with the Ni/(Ni + Mo) atomic ratio, and reaches the maximum when the Ni/(Ni + Mo) atomic ratio is near 0.60. XRD characterization shows that the catalyst phases are in nitrided forms, such as Mo2N and Ni3Mo3N The high activity of the nitrided MoNx/alpha-Al2O3 and NiMoNy/alpha-Al2O3 catalysts is attributed to the nitride phases formed from the preparation and the reaction in the presence of ammonia.