In the present investigation, carbon-expanded α (αC) phase layers were prepared by low-temperature carburizing of AISI 431 martensitic stainless steel. The microstructures of the layers were characterized by optical microscopy and X-ray diffractometry, the mechanical properties of the carburized layers were characterized by nanoindentation tester. Then, first-principles calculation method was used to characterize αC phase from atomistic level. Experimental results show that there are the uniform layers formed on stainless steels after plasma carburized at 703 K. The layer thicknesses for the steel carburized for 14,400 s, 28,800 s and 57,600 s are about 25 μm, 38 μm and 46 μm, respectively. The surface phases are mainly of αC phase with a small amount of Fe3C phase. The surface hardness of the carburized specimen can be higher than 15 GPa by nanoindentation tests, the carburized layers possess a good wear resistance based on the higher H/E and H3/E2 ratio evaluation criterions. Crystal cell of αC phase were established and describing as the nominal molecular formula of Fe13Cr3Cx (x = 0, 2, 4, 6). The calculation results indicate that with increasing of carbon content, the structural and mechanical stabilities of Fe13Cr3Cx (x = 0, 2, 4, 6) system decrease gradually, the hardness increases first then decrease, and can reach up to about 25 GPa. The elastic moduli decrease from about 210 GPa to 66 GPa with increasing of carbon content, while the toughness increase gradually based on B/G values evaluation criterion. The calculated and the experimental results are consistent within a reasonable range, which shows the validity of the established model for αC phase.