By using the Fick’s second law,the carbon concentration of phaseγat non-equilibrium interface and its distribution in austenite during the decomposition of austenite to ferrite in Fe-C alloys were calculated.For example,it takes 160 s at 1 025 K,210 s at 975 K,270 s at 925 K and 580 s at 875 K for reaching the equilibrium carbon concentrations of phase y at theα/γinterface in Fe-0.2C alloy,and the relationship between the ratio C_(int)~γ/C~(γ/α)and the time meets the parabola law with parabola exponents of 0.167 39,0.235 97,0.319 87 and0.389 44 at 1 025 K,975 K,925 K and 875 K respectively.The decrease of the diffusion velocity of carbon in austenite with the increase of under-cooling would be counteracted by the increase of driving force for pro-eutectoid ferrite formation before soft impingement of ferrite grains with each other;but the effect of diffusion is more intensive than that of driving force at the later stage. By using the Fick’s second law, the carbon concentration of phase γat non-equilibrium interface and its distribution in austenite during the decomposition of austenite to ferrite in Fe-C alloys were calculated. For example, it takes 160 s at 1 025 K, 210 s at 975 K, 270 s at 925 K and 580 s at 875 K for reaching the equilibrium carbon concentrations of phase y at the α / γinterface in Fe-0.2C alloy, and the relationship between the ratio C_ (int) ~γ / C ~ (γ / α) and the time meets the parabola law with parabola exponents of 0.167 39,0.235 97,0.319 87 and0.389 44 at 1 025 K, 975 K, 925 K and 875 K respectively. The decrease of the diffusion velocity of carbon in austenite with the increase of under-cooling would be counteracted by the increase of driving force for pro-eutectoid ferrite formation before soft impingement of ferrite grains with each other; but the effect of diffusion is more intensive than that of driving force at the later stage.