Due to environmental constraints, sulfur content of diesel fuel has been restricted to very low levels (500 ppm maximum) in many countries. As a result, a greater emphasis has been placed in recent years on the development of catalysts and processes for deep desulfurization of diesel blending streams to produce low sulfur diesel fuel. In the present work we have compared the performance of a conventional Co-Mo catalyst with that of high metal loading Co-Mo and Ni-Mo catalysts in deep desulfurization of Kuwait atmospheric gas oil. The tests were carried out in a fixed bed reactor unit using 75 ml of catalyst under the conditions: P=32 bar; LHSV = 4h−1; H2/oil ratio = 100 ml/ml; temperature range = 330 − 390°C. HDS activity of the high molybdenum Co-Mo catalyst was superior to that of the conventional Co-Mo hydrotreating catalyst. High metal loading Co-Mo/Al2O3catalyst also showed a substantially higher HDS activity than the Ni-Mo/Al2O3catalyst containing a similar high metal loading. The unreacted sulfur compounds remaining in the product after high severity hydrotreating were identified as dibenzothiophenes with alkyl substituents next to the sulfur atom. The desulfiirization of such low reactive alkyl dibenzothiophenes was found to occur at a substantially lower temperature over the high metal loading Co-Mo catalyst compared with the conventional Co-Mo catalyst. The results have been explained on the basis of the stacking and dispersion of MoS2slabs as well as in terms of the nature of the sulfur vacancies in the MoS2layers in these catalyst systems.