Yttrium-coated Ni{sub 3}Al with post heat treatment has shown good high-temperature oxidation resistance. To understand the effect of the Y-coating and post heat treatment on the oxidation resistance of Ni{sub 3}Al, the specimens were coated with Y by an ion-plating method, and heat treatment was performed at low oxygen level before or after the Y-coating was applied. Performance of the Y-coated Ni{sub 3}Al was evaluated by isothermal and cyclic oxidation tests. A simple deposition of Y on Ni{sub 3}Al did not change the oxidation kinetics, but the post heat treatment after Y-ion plating significantly decreased the oxidation rate of Ni{sub 3}Al. The scale formed on Y-coated Ni{sub 3}Al with post heat treatment after Y-ion plating showed a fine and dense structure which was grain refined by the presence of a (Y, Al) O-type oxide in the scale. The coated Y layer becomes a Y-Al compound during heat treatment. The presence of the (Y, Al) O-type oxide in grain boundaries or the lattice of Al{sub 2}O{sub 3} modify the diffusion rate of Al and Oxygen, and the oxide microstructure during oxidation. Improvement of cyclic-oxidation resistance of Ni{sub 3}Al by the Y-coating occurs because the presence of (Y, Al) O-type oxide developsmore » fine-grain oxides which can easily relieve the growth stress.« less