High temperature behaviour of yttrium implanted pure iron and extra low carbon steel were analyzed at T = 700 degrees C and under oxygen partial pressure P-O2 = 0.04 Pa for 24 h to show the benefit of yttrium incorporation to the improvement of the implanted sample corrosion resistance at high temperature. Compositions and structures of yttrium implanted and unimplanted samples were investigated prior to high temperature oxidation studies by several analytical and structural techniques (RBS, RHEED and XRD) to observe the initial yttrium implantation depth profiles in the specimens. High temperature oxidation tests performed by thermogravimetry and by in-situ high temperature X-ray diffraction were carried out with the same experimental conditions on yttrium implanted and unimplanted samples to have reference analyses. The aim of this paper is to show the initial nucleation stage of the main compounds induced by oxidation at high temperature according to the initial sample treatment. The results obtained by in-situ high temperature X-ray diffraction will be compared to those by thermogravimetry to show the existing correlation between weight gain curves and structural studies. Our results allow to understand the improved corrosion resistance of yttrium implanted pure iron and extra low carbon steel at high temperature.