The adsorption and dissociation of NH 3 on Ir(100) have been investigated using the periodic density functional calculations. The recombination desorption of N 2 has also been studied. The corresponding reaction energies, the structure of the transition states and the activation energies were determined. The calculated activation barrier for NH x (x=1–3) dehydrogenation is between 0.83 and 1.09eV including the zero point energy correction. The NH 3 desorption energy of 0.95eV is close to the NH 3 dehydrogenation barrier of 0.91eV, which indicates that the desorption and dissociation of NH 3 on Ir(100) is very competitive, consistent with the recent experimental results. The N–H bond cleavage in NH 3 is found to be the rate limiting step. The activation energy for the recombinative desorption of N 2 is significantly lower than those for the NH x dehydrogenation. Moreover, it was found that the zero point energy has a large contribution to the reaction energies and activation barriers.