Abstract: Coke on spent catalysts from various stages of a commercial fixed-bed hydrotreating unit processing residue feed was characterized by SEM, TPO-MS and CHO elemental analyzer. Test results show that following the feed flow direction, the coke contents of catalysts exhibit a trend of firstly decrease then increase, the H/C atomic ratios of coke show decrease trend and oxidation temperatures of coke increase gradually. It is shown that the formation of coke is related with the nitrogen containing compounds of feed in some ways, asphaltene containing nitrogen atom is easy to be adsorbed on the acid sites of catalyst and to form coke further.