Yttrium implanted and unimplanted low manganese and low manganese-carbon steel samples were analyzed at T = 700 degrees C and under an oxygen partial pressure P-O2 = 0.04 Pa for 24 h to show the yttrium implantation effect on sample high temperature oxidation resistances. Sample oxidation weight gains were studied by thermogravimetry and structural analyses were performed by in-situ high temperature X-ray diffraction (XRD) with the same experimental conditions. 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 XRD will be compared to those by thermogravimetry to a better understanding of the weight gain curves observed during the oxidation test at 700 degrees C and P-O2 = 0.04 Pa. This study allows us to show the improved corrosion resistance of yttrium implanted low manganese and low manganese-carbon steel at high temperature.