In order to investigate the effect of lanthanum ion imprantation on the oxidation behavior of zircaloy at 500℃, Zircaloy specimens were implanted by lanthanum ions with a dose range from 5×10¹⁶ to 2×10¹⁷ ions/cm² at room temperature, and then oxidized at 500℃ for 100 min. The valence of the oxides in the scale was analyzed by X-ray Photoelectron Spectroscopy (XPS). The phase structures of the oxides in the scale were examined by Glancing Angle X-ray Diffraction (GAXRD). With the increase of implanted lanthanum ions dose, the phase structures in the oxide scale are transformed from monoclinic zirconia to hexagonal one and then to monoclinic one again. The measurement of weight gain showed that a similar change from the decreased gain to increased one again is achieved in the oxidation behavior of lanthanum ion implanted zircaloy compared with that of as-received zircaloy.