A new hydrodemetallization (HDM) catalyst with low catalytic activity was prepared to accomplish the hydrotreating of atmospheric residue (AR) under the deep desulfurization condition (sulfur content in product oil is below 0.3 wt.%). The combination of the HDM catalyst and the hydrodesulfurization (HDS) catalyst supported on an alumina with medium pore (11 nm) was applied to the hydrotreating of the AR to elucidate the effect of chemical composition of the HDM catalyst on the catalyst life. The operation, exceeding 6000 h, was accomplished over the combined catalyst system under the deep desulfurization conditions. The preferential decomposition of maltene as the heavy residue with high reactivity on the low active HDM catalyst resulted in the long catalyst life. The hydrotreating of atmospheric residue was carried out to elucidate the effect of the reaction temperature of the guard reactor on the life of HDM and HDS catalysts. When the guard reaction temperature was raised to 683 K, the catalyst was rapidly deactivated due to the deposition of coke on the catalysts. These results suggest that the separate decomposition of maltene and asphaltene is important for continuous hydrotreating of atmospheric residue. (c) 2005 Elsevier B.V. All rights reserved.