The p(2 X 3) Ni(110)-N surface structure was produced by the reaction of NO with highly excess H-2 (1:150) on a Ni(110) surface at 650 K. The N atoms on Ni(110) surface were inactive for the hydrogenation with H-2 They, however, reacted with H atoms in the presence of H-2, and the formation of NH species was detected by high resolution electron energy loss spectroscopy (HREELS). In the temperature range between 300 K and 450 K, the amount of N on Ni(110) surface decreased by reacting with H atoms in the zero order kinetics with respect to the amount of N and the decreasing rate did not depend on the temperature. When the temperature is higher than a critical temperature of 500 K, however, the amount of N on the Ni(110) surface does not decrease even if the surface is exposed to H atoms. This critical temperature corresponds to the decomposition temperature of the NH species on Ni(110) surface determined by the HREELS. One one hand, the NH species were produced by the reaction, hydrogenation, of N with H atoms on the Ni(110) suface. On the other hand, it was reported that NH2 species were preferentially formed by the decomposition, dehydrogenation, of NH3 on the Ni(110) surface. Taking these facts into account, it was deduced that there is a rather high activation barrier in an elementary reaction path from NH to NH2 on the Ni(110) surface.