The up-hill diffusion process was proposed to form a nickel aluminide coating on TiAl alloys by electroplating a Ni film followed by a high Al activity pack cementation. The coating has a duplex layer structure, an outer Ni₂Al₃ layer and an inner TiNiAl₂ /TiAl₃ / TiAl₂ layer.　The coated TiAl was oxidized in air for long terms (up to 36,000ks (10,000hrs)) under thermal cycling between room temperature and 1173K. A protective Al₂O₃ scale formed with little oxide exfoliation and the oxidation amount was 37 g/m² after the 36,000ks oxidation. During oxidation at 1173K the outer Ni₂Al₃ changed to the β-NiAl with voids and then the β-NiAl / TiNiAl₂, and the inner TiNiAl₂ / TiAl₃ / TiAl₂ layer to the TiNiAl₂ / TiAl₂ layer and then the TiNiAl₂ / TiAl₂ /τ₃ layer. The voids in the outer layer were formed by the phase transformation from the Ni₂Al₃ to β-NiAl during oxidation. It was found that the higher Al contents in the inner layers were retained than that of the outer layer after 36,000ks oxidation. The changes in a layer structure the nickel aluminide coating is discussed, based on diffusion and composition paths plotted on the Ti-Al-Ni ternary phase diagram at 1173K.