Abstract: A mathematical model of kinetics of carbide layer formed on the surface of steel in fused system was set up with the theory of non-stable diffusion mass transfer and regular solution sub-lattice. This model was analyzed and testified by the experiments in which the VC layers were formed on four kinds of steels as steel 45,4Cr5MoSiV1,T10, and Cr12MoV immersed in the V₂O₅-Na₂O-B₂O₅ fused system with the reducing agent of silicon.The results showed that there was a good agreement between the model and the experimental results;the growth speed of carbide layer depended on the factors of carbon activity in substrate, treatment temperature,kinds of carbides and its structure etc.