To study the effect of pore size distribution (PSD) of Ni-Mo/Al2O3 catalysts on the hydrodemetallization (HDM) activity, catalysts with different PSD were prepared using activated carbon and ammonium bicarbonate as pore-expanding agents and the HDM reaction was carried out in a high pressure trickle bed reactor and Saudi Arabia vacuum residuum as the feedstock. The catalysts showed good activity of removing metal nickel (Ni) and vanadium (V) when they contain 10-20 nm pores, but the catalysts with both 10-20 and >100 nm pores have higher activity of removing metal V. The effect of PSD on the removal of different metal compounds were also studied by analyzing different metal compounds distribution in its hydrotreated products. Both Ni and V compounds of small molecular size in the methanol extract and middle molecular size in the dimethylformatmide (DMF) extracts can be easily removed on the catalysts. However, most of the large molecular V and Ni compounds in the toluene extracts were the most resistant to be removed in the vacuum residuum hydrodemetallization. Large molecular V compounds in the toluene extracts can be hydrodemetallized effectively on the catalysts with macropore size (>100 nm) distribution. While catalysts with mesopore size distribution (10-20 nm) shows a higher HDNi activity for the large molecular Ni compounds in the toluene extracts. Catalyst with both 10-20 and >100 nm pore size distribution would be more appropriate for the Saudi Arabia vacuum residuum HDV and HDNi. (C) 2015 Elsevier B.V. All rights reserved.