In order to increase the oxidation properties at elevated temperature, Pt-Al coating has practically been in applied for Ni-based superalloys. The Pt-Al coating, however, suffers from erumpling' problems, mainly induced by the phase transformation when Al is consumed during service. On the other hand, Ir-Al phase diagram does not show any intermediate phases between IrAl b-phase and the Ir-based solid solution phase, thus Ir-Al-based coatings are expected to be more stable than Pt-Al in terms of phase transformation. However, there are very limited data for the Ir-Ni-Al alloy system reported so far. The main purpose of this study is to understand the diffusion kinetics in the Ir-Ni-Al alloy system. Ni(gamma)/Ir,Ni24at%Al(gamma')/Ir and Ni16at%Al(gamma+gamma')/Ir diffusion couples were made to understand the diffusion behavior of Ir in Ir-Al coated Ni-base superalloys.. These diffusion couples were annealed in a vacuum furnace at 1273-1473K for 5-25hours. Cross-sections of annealed samples were analyzed by SEM and EPMA and following results were obtained: 1) In both Ni24at%Al/Ir and Ni16at%Al/Ir diffusion couples (Ir,Ni)0.5 Al0.5 beta phase is formed by reactive diffusion of Ir and Al, which is difficult to predict from the Ir-Ni-Al phase-diagram. 2) Al concentration drops in the vicinity of beta/gamma' and gamma+gamma' interfaces, followed by the uphill diffusion shown in the b phase. 3) Diffusion of Ni into Ir is much faster than that of Ir into Ni. These results confirmed that .the interaction between Ir and Al is so strong that they easily form (Ir,Ni)Al beta phase, and the formation of the b-phase can be caused by the fast diffusion of Ni into Ir, which accordingly increase the local Al concentration at the NiAl/Ir interface.