Kinetics and microstructure development on Sulfonitriding of die- steels were discussed. Samples made of a die-steels, Fe-5 mass%Cr-5mass%V-lmass%Si, JIS-SKD61) was treated at 520°C in the stream of H2S amd NH3 containing gas mixture. The top layer made of dense FeS was formed on the second layer of iron nitride on the diffusion zone of Fe(N) solid solution. Results of the marker experiments with AI2O3 powder reveals that outward diffusion of Fe occurs for the growth of the top layer. The second layer and the diffusion zone grow via inward diffusion of N.