In order to investigate the behavior of sulfur on sulfided Ni-Co-Mo/Al2O3 catalyst in the ultra deep hydrodesulfurization (HDS), a novel radioactive S-35 tracer method was developed. Both radioactive S-35-labeled elemental sulfur and dibenzothiophene (DBT) were used to label the catalyst under the practical HDS, and then HDS reactions of DBT of 0.001-1.0 wt.% (content of sulfur: ca. 2-200 ppm) were carried out over the S-35-labeled catalyst at 260 degreesC and under 5.00 MPa. By tracing the changes in the radioactivity of formed [S-35]H2S, the behavior of sulfur on the catalyst was determined. Either [S-35]S or [S-35]DBT can be used to label the catalyst well and the behavior of sulfur on the catalyst did not change with the type of S-35 agent. The release rate of [S-35]H2S from S-35-labeled catalyst in the HDS of DBT became remarkably slow when the concentration of DBT decreased to less than 0.01 wt.%. The amount of labile sulfur participating in the HDS of DBT also decreased with decreasing the concentration of DBT. This indicates that the labile sulfur on the catalyst is not uniform and only some part but not all of labile sulfur participates in the ultra deep HDS.