Iron catalyst for ammonia synthesis of various mean sizes of iron nanocrystallites were nitrided with ammonia in a differential reactor equipped with systems that made it possible to conduct both thermogravimetric measurements and hydrogen concentration analyses in the reacting gas mixture. The nitriding process was investigated under atmospheric pressure at the temperature of 475°C. It was found that along with an increase of mean size of iron nanocrystallites, with a decrease of specific surface area of the samples, nitriding degree of solid samples increased. At the same time the rate of surface reaction of catalytic ammonia decomposition decreased. Along with an increase of the samples’ specific surface area an increase of the catalyst’s activity was observed. However, it was also observed that the concentration of active sites on the catalysts’ surface decreased along with an increase of specific surface area.