The selective hydrogenation of halogenated nitrobenzene over noble metal catalysts(Pd, Pt, and Ir) has attracted much attention owing to its high efficiency and environmental friendliness. However, the effect of size on the catalytic performance varies among different metal catalysts. In this study, sub-nano(<3 nm) Ir and Pd particles were prepared, and their catalytic properties for hydrogenation of halogenated nitrobenzene were evaluated.Results show that high selectivity(N 99%) was achieved over small Ir nanoparticles, in which the selectivity over the Pd with same size was much lower than that on Ir nanoparticles. Meanwhile, Ir and Pd have different hydrogen consumption rates and reaction rates. Density functional theory calculations showed that p-chloronitrobenzene(CNB) has different adsorption properties on Ir and Pd. The distance between oxygen(cholorine) and Ir is much shorter(longer) than that between oxygen and Pd. The reaction barriers of dechlorination of p-CNB and p-chloroaniline over different Ir models are much larger than those on Pd. Especially,lower coordination of Ir leads to larger barriers of dechlorination reaction. These theoretical results explain the difference between Ir and Pd on hydrogenation of halogenated nitrobenzene. The selective hydrogenation of halogenated nitrobenzene over noble metal catalysts (Pd, Pt, and Ir) has attracted much attention owing to its high efficiency and environmental friendliness. However, the effect of size on the catalytic performance varies among different metal catalysts. , sub-nano (<3 nm) Ir and Pd particles were prepared, and their catalytic properties for hydrogenation of halogenated nitrobenzene were evaluated. Results show that high selectivity (N 99%) was achieved over small Ir nanoparticles, the Pd with the same size was much lower than that on Ir nanoparticles. Also, Ir and Pd have different hydrogen consumption rates and reaction rates. Density functional theory calculations showed that p-chloronitrobenzene (CNB) has different adsorption properties on Ir and Pd. distance between oxygen (cholorine) and Ir is much shorter (longer) than that between oxygen and Pd. The reaction barriers of dechlorination of p-CNB and p-chloro Aniline over different Ir models are much larger than those on Pd. Especially, lower coordination of Ir leads to larger barriers of dechlorination reaction. These theoretical results explain the difference between Ir and Pd on hydrogenation of halogenated nitrobenzene.