Effect of high-energy (3.8 MeV) nitrogen ion implantation on the rate of thermal oxidation of Zr-2.5%Nb alloy at the temperature of 400°C in air has been investigated. Pure nonoxidized as well as oxidized on parabolic and linear stages of oxidation kinetics specimens were implanted. The degree of decreasing the oxidation rate is shown to be substantially dependent on both the ion fluence and specimen conditions during implantation. The effects observed are interpreted in terms of the Wagner's theory of high-temperature oxidation.