The spheroidization of cementite in ternary Fe-C-Si alloy was investigated at temperatures of 600, 640 and 680℃ for times up to about 10^6 sec. The mean size and the number of particles per unit volume were obtained by quantitative metallography techniques, and analyzed by means of Lifshitz-Wagner model assuming diffusion-controlled mechanism. The effective diffusion coefficient was between the values for the diffusion of carbon and ron in ferrite and way clone to the coupled diffusion coefficients developed by Oriani and Li, Blakely and Feingold. The effective diffusivity is 5.6×10^(-11)㎠/sec at 680℃.