Abstract: The effect of hydrogen ion implantation on the microstructure ofODS ferritic steel was studied by means of ion implantation at room temperature and in-situ observations in a low voltage electron microscope. The results indicated that a certain amount of (Fe,Cr)₂O₃ phases existed in the original ODS alloy. They did not change obviously after hydrogen ion implantation at room temperature. During heating the specimen implanted with hydrogen ions, however, the (Fe,Cr)₂O₃ phase dissolved gradually and the composition of the residual (Fe,Cr)₂O₃ phase changed too. On the contrary, the microstructure of the specimen which was not implanted with hydrogen ions did not changed obviously during heating, meanwhile, (Fe,Cr)₂O₃ phase did not dissolve.