Yttrium implanted and unimplanted pure iron oxidation behaviour were studied by thermogravimetry, scanning electron microscopy (SEM) and in situ X-ray diffraction at T=700°C and P O2 =0.04 Pa for 24h to determine the yttrium implantation effect on sample oxidation resistance at high temperature. In situ X-ray diffraction analyses performed on unimplanted pure iron allow to observe the initial nucleation stage of FeO (wustite), Fe 3 O 4 (magnetite) and Fe 2 O 3 (hematite) whereas two other compounds (i.e. FeYO 3 and Fe 2 YO 4 ) were detected in combination with iron oxides in the case of yttrium implanted pure iron. These two yttrium-iron mixed oxides seem to be responsible for the low oxidation rate observed by thermogravimetry. This paper shows that in situ high temperature X-ray diffraction technique provides a better understanding of yttrium implanted and unimplanted pure iron oxidation mechanisms.