Periodic DFT calculations have been performed to systematically investigate the catalytic mechanisms of ammonia (NH3) decomposition on clean, O- and OH-assisted Ir(1?1?1) surfaces. The adsorption configurations, reaction energies and barriers, and elementary steps were elaborated. Our results show that the NHx (x=1–3) decomposition prefers to proceed by means of the O- and OH-assisted reaction mechanisms, NH3+O→NH2+OH, NH2+OH→NH+H2O, and NH+OH→N+H2O, rather than the direct NHx decomposition of NH3→NH2→NH→N as a result of the high energy barriers involved. The promotion effect of the O- and OH-oxidizing agents are then discussed using energy barrier analysis. The relationships between the selectivity toward the final product and coverage, O to N coverage, and reaction temperature are elucidated. Finally, we compare our results with analogous investigations of NH3 decomposition on Pt, Rh, and Ir surfaces.