A series of coked catalysts were prepared with Co-Mo/γ-Al₂O₃ hydrotreating catalyst by reacting with vacuum gas oil. The physical properties and catalytic activities of these coked catalysts were examined. A similar series of coked catalysts were prepared wish carbazole added to vacuum gas oil, and the changes of physical properties and catalytic activities by nitrogen compounds were also examined. It was found that the amount of coke deposited on the surface of catalyst increases with increase of reaction temperature and reaction time. As amounts of coke increases, pore volume and surface area decrease significantly. The order of reactivity of the fresh catalyst was : HDS > CNH >HYD >HDN. As the amount of coke increases, VGO-coked catalyst shows deactivation as order of HDN > CNH > HYD > HDS. Carbazole-coked catalyst shows much more deactivation in all reaction fuctionalities compared to the VGO-coked catalyst. Surface area and pore volume of carbazole-coked catalyst decrease much more than VGO-coked catalyst. These results can be explained as the server plugging of pore mouth by the deposition of nitrogen compounds.