Three catalysts namely NiMo/Al2O3, NiMoB/Al2O3 and NiMoP/Al2O3 were prepared, extensively characterized and were used for hydrodenitrogenation (HDN) and hydodesulfurization (HDS) of heavy gas oil obtained from Athabasca bitumen. The hexagonal structure of MoS2 of the catalyst at different stages of reaction was studied and quantified using transmission electron microscopy (TEM). The length and number Of MoS2 stack, the dispersion of edge and corner molybdenum atoms, number Of MoS2 stacks and number Of MoS2 layers per stack, which play important role on the HDN and HDS activities of these catalysts, were calculated. It was concluded that the decrease in MoS2 slab length caused an increase in catalyst activity for HDN of heavy gas oil derived from Athabasca bitumen. Computer simulation was also performed in order to correlate the surface morphology with catalyst activity. Maximum edge and corner Mo atoms which were responsible for high HDN and HDS reactions, was obtained at an average MoS2 slab length of approximately 20 and 10 angstrom, respectively. (c) 2005 Elsevier B.V. All rights reserved.