Deep HDS of LCO to be <10 ppm was achieved by fractionating HDS. An LCO fraction having boiling point lower than 340 degreesC was easily hydrodesulfurized to be <10ppmS under conventional HDS conditions because the hydrodesulfurization of the reactive sulfur species contained in the fraction was hardly inhibited by the aromatic compounds. In contrast, the heavier fraction was very hard to be hydrodesulfurized since its refractory sulfur species were strongly inhibited by large content of aromatic components. Dilution of the heavier fraction with common solvents showed higher reactivity over NiMo/Al2O3 catalyst than itself alone. Decalin, which showed the highest dilution effect, was believed to strip the strongly adsorbed aromatics off from the catalyst surface. (C) 2004 Elsevier B.V. All rights reserved.