Objective To investigate the sulfidation mechanism of FV(520)B stainless steel at primary stage. Methods The sulfidation specimen was prepared by the exposure of FV(520)B stainless steel to the atmosphere of 1. 5 MPa H2S at 300℃. XPS and SEM analysis techniques were applied to investigate the content, the distribution of element and the composition of the sulfide scale. Results The multilayered sulfide scale was composed of an outer layer of Fe7S8,FeS2, Ni(1-x)S and a inner layer of Fe7S8, FeS2, Ni(1-x)S, Cr2S3. XPS data indicated that Fe3+ and Fe2+ were found in Fe7S8 and FeS2 in all sputtered layers, and the average contents of Fe3+ were 36. 53% and 35. 78% , respectively. Ni2+ and Ni3+ were also observed in Ni(1-x)S and the content of Ni3+ was 32. 08% . Conclusion The data analysis demonstrated that the cation vacancy was the predominant defect in ferrous, nickel sulfide and the defect concentrations were high. Outward diffusion of Fe and Ni ions were constrained by Cr2S3 in the inner layer, but the inhibitory effect was not obvious due to the lack of Cr2S3 in sulfide scale at the primary stage of sulfidation.