High temperature X-ray diffraction( HTXRD) was employed to investigate phase transformations of the thermally grown wustite ( Fe1-xO) within the oxide scale.The quantitative analysis of the relative phases,wustite,magnetite( Fe3O4 ),hematite ( a-Fe2O3 ) and α-Fe,during the decomposition of wustite in oxide scale at 550,500 and 450℃ were obtained by use of linear position sensitive detectors.The HTXRD results show that the diffraction patterns of eutectoid products( magnetite and α-Fe) develop with oxidation time and temperature with various thickness of oxide scale.The eutectoid transformation balances during the long period of time,with reflections corresponding to more intensity of magnetite than that of a-Fe.Eutectoid products in the oxide scale for thickness more than 15 μm have a decrease with the increase of temperature.However,for hematite it increases.The HTXRD gives potential insights on the phase evolution of oxide scale occurring after hot rolling,and it also provides the preliminary profiles of grain-size distribution for the analysis of the grain orientations.