The influences of nitrogen implantation on the corrosion resistance of Zircaloy-4 were examined by immersion tests in high-temperature water at 350°C, and the results were discussed with structural and compositional variations of implanted layer that were measured by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The nitrogen-implanted layer was composed either of ZrN and ZrO 2 layers or of nitrogen-implanted layer with ZrO 2 layer whether the implanted nitrogen ion dose is greater than the critical value of about 10 18 ions cm −2 necessary for the formation of ZrN layer or not. It was found that most of the nitrogen implanted into Zircaloy-4 was released into water when exposed to the high-temperature water, presumably by a decomposed reaction of ZrN by oxygen diffused from water, thereby producing ZrO 2 layer the thickness of which was very sensitive to both the substrate temperature and the nitrogen ion dose. The corrosion resistance of the nitrogen-implanted Zircaloy-4 was improved with increasing the ion dose, provided that the ion dose is less than the critical value for the formation of a continuous ZrN layer. The best corrosion resistance of Zircaloy-4 was obtained when implanted with a nitrogen ion dose of 5×10 17 ions cm −2 at a substrate temperature of 400°C due to an increased thickness of ZrO 2 layer.