Deep hydrodesulfurization (HDS) of gas oil has become one of the most important subjects in the production of fuels, as increasingly stringent quality standards for gas oil have been introduced all around the world. We studied the technical conditions (temperature, hydrogen pressure, liquid hourly space velocity, and H-2/oil volume ratio) for the deep HDS of fluid catalytic cracking (FCC) and coking gas oils over a highly loaded NiMoW catalyst and compared its activity and the removal of the refractory dibenzothiophenes (DBTs) with those of a commercial NiMo/gamma-Al2O3 catalyst and a highly loaded NiMo catalyst. The FCC gas oil, which had a lower sulfur content and simpler composition than the coking gas oil, was relatively easy to process. The coking gas oil and another FCC gas oil, of poor quality with more complex compositions, required more stringent reaction conditions. The sulfur distributions in gas oils hydrotreated to a level of 100, 50, and 10 mu g/g sulfur were investigated. 4-Methyldibenzothiophene could be totally removed when the coking gas oil was hydrotreated to a level of 50 mu g/g S. Only 4,6-dimethyldibenzothiophene and 2,4,6-trimethyldibenzothiophene were detected in the deep HDS to a level less than 10 mu g/g S. This is attributed to the steric hindrance of substituted-alkyls at the 4 and/or 6 positions of DBT. The highly loaded NiMoW catalyst had a higher activity than a highly loaded NiMo catalyst and a commercial NiMo/gamma-Al2O3 catalyst, and was more suitable for the production of ultra-low-sulfur gas oils. (C) 2015 Elsevier B.V. All rights reserved.