Decarburization phenomena were investigated at 800 ℃ by the PH₂O/PH₂＋Ar gas mixture in the case iron range which contains Si, Mn and Cr as an alloying elements. Dissociation of cementite in a matrix which contains graphitizer as Si begins at the carbon rich cementite dendrite arms. Several primary austenite (γ) skeletons are surrounded by those nucleated graphite nodules, and that forms a limited area of nucleation region. Decarburization reactions at 800℃ in Fe-C, Fe-Mn-C and Fe-Cr-C alloy are followed by parabolic rate law under the gas mixture of PH₂O/PH₂=0.01 and the modified rate const. k were in the range of 1∼6×10＾(-10)㎠/s.