A refined method for analysing X-ray diffraction spectrum was used to determine the carbon content bainite ferrite in 40CrMnSiMoV steel. The supersaturated carbon concentration linearly decreases with increase of isothermal holding time at 310 degrees C due to the carbon repartition and carbide precipitate The deduced carbon content of lower bainite embryo in the steel is 0.36 wt%, which agrees well with prediction from the mechanism that bainite forms in carbon depleted regions in supercooling augment through martensitic shear. (C) 2009 Elsevier B.V. All rights reserved.