Yttrium implanted and unimplanted extra low carbon steel and pure electrolytic iron samples were investigated at high temperature (T=700 degrees C) under oxygen partial pressure P-O2 = 0.04 Pa to observe their oxidation resistances under these experimental conditions. High temperature oxidation tests were carried out by thermogravimetry and by in-situ high temperature X-ray diffraction. 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 (yttrium implanted or unimplanted). 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 resistances of yttrium implanted extra low carbon steel and pure iron at high temperature.