The isothermal diagrams of the 12% Cr-Fe-C-N system in the composition range up to 0.4% carbon and 0.3% nitrogen were studied at temperatures from 1300℃ to 700℃. Resorting to the consideration of phase relationship, the phase reaction of 12%Cr-Fe-C-N system was clarified and the sectional diagrams were constructed at fixed contents of 0.1% and 0.2% nitrogen and carbon, respectively. In Fe-Cr-C-N system, the quaternary peritecto-eutectoid reaction (chemical formula) exists at about 780℃, representing by the hexahedron of five-phase region consisting of α, γ, Cr_C_6, Cr_7C_3 and Cr_2N. The Cr_7C_3 type carbide detected in the present work is expressed by the formula (Fe_2C_5)C_N_
