Abstract: The effectiveness of small amount silicon (1,5%, 2.5%) on the high temperature sulfidation properties of Fe-25Cr alloy in H₂S-H₂ mixtures has been investigated. The addition of silicon enhanced the sulfidation resistance of Fe-25Cr alloy by varying degree above 800℃. The sulfidation kinetics of alloys with Si followed the parabolic law unperfectly because of the cracking of sulfide scale during sulfidizing process. A dense and integrated inner-layer but of poor protection formed on the alloys bearing Si. On the basis of analysis of sulfidation kinetics curves, structure and construction of sulfide scale, uhe sulfidation mechanism of Fe-25Cr-(1.5%, 2.5%) Si alloys has been proposed and the effectiveness of Si discussed.