The present study confirmed that the removal of both nitrogen and refractory sulfur species in the gas oil was very effective to achieve its sulfur level of less than 10 ppm by the conventional HDS over CoMoS/silica-alumina catalyst. The sulfur content in HDS product was governed by the nitrogen species in feed oil when its content was more than 60 ppm whereas it reflected the content of the refractory sulfur as well as that of nitrogen species in feed oil when that of nitrogen species in feed oil was less than 60 ppm. Langmuir-Hinshelwood type equation was assumed to describe the contribution of nitrogen and refractory sulfur species to the achievable sulfur level of HDS. The inhibiting terms included in the equation were estimated to 0.75 order of the initial content of nitrogen and first order of refractory sulfur species, respectively. Thus the achievable sulfur level of HDS product from feeds with different content of nitrogen and refractory sulfur species is calculated by the equation. (C) 2004 Elsevier B.V. All rights reserved.