To discuss the potential role of iridium (Ir) nanoparticles loaded under atmospheric conditions, we prepared a series of catalysts with the same active phase but different contents of 10wt%, 15wt%, and 20wt% on gamma-alumina for decomposition of hydrazine. The performance of the catalysts was better with 15wt% and 20wt% of the Ir nanoparticles, and also the selectivity to hydrogen was about 27%. An increase in the reaction rate from 181 h-1 to 218 h−1 was observed in the loading of 15% by weight of iridium particles due to the good dispersion of the active phases by preventing surface agglomeration. As a satisfactory result of this investigation, Ir catalysts with different weight percentage (15wt% and 20wt%) showed the same activity and selectivity, and are suitable substitutes for each other. Using a catalyst with a lower weight percentage of the active phase and high activity is economically acceptable due to its low cost.