The effect of Y addition on the microstructure and isothermal oxidation behaviour of an equal-atomic Ti-Ni shape memory alloy at 700 ℃ in air was investigated. The results showed that the microstructure of Ti₅₀Ni₅₀ alloy was changed obviously with Y addition and the grains of Ti-Ni alloy was evidently refined. The oxidation rate was significantly reduced by the addition of 0.5 at. %Y and 1.0 at. %Y due to the inhibition of the fast outward diffusion of Ti by the segregated Y ions. However, the oxidation rate was increased by 5 at.%Y addition since the formation of oxide along the Y-rich phase broke the continuity of the TiO₂ scale.