The catalytic ammonia decomposition over iron and iron nitride, Fe4N, under the atmosphere of ammonia-hydrogen mixtures of different amounts of ammonia in the temperature range of 400-550 A degrees C by means of thermogravimetry has been studied. A differential tubular reactor with mixing has been used. The ammonia concentration in the gas phase during all the process was analysed. The balance between the inlet and outlet ammonia quantity has been used to determine a degree of ammonia conversion and the values of decomposition reaction rate. The activation energy of ammonia decomposition reaction over Fe and Fe4N was found to be 68 and 143 kJ/mol, respectively.