PURPOSE: To convert a heavy hydrocarbon oil to a highly value-added hydrocarbon oil and thereby to reduce or remove asphaltene, heavy metals, sulfur, etc., by hydrotreating the heavy hydrocarbon oil with a catalyst comprising a specified clay mineral, pseudoboehmite and a specified metal component.

CONSTITUTION: A heavy hydrocarbon oil is reduced in content of asphaltene, heavy metals, sulfur, etc., and thereby converted into a highly value-added hydrocarbon oil, by catalytically hydrotreating the heavy hydrocarbon oil under reaction conditions including a hydrogen/oil ratio of 100W2,000Nl/l, a temperature of 350W 450°C, a pressure of 30W250Kg/cm2.G and a liquid hourly space velocity of 0.1W 10hr-1, in the presence of a catalyst comprising a double chain-structured clay mineral consisting essentially of magnesium silicate; pseudoboehmite showing in its powder X-ray diffraction spectrum (Cu Kα rays), a half width of the peak for face (020), of about 0.8W4° and an intensity of the peak about 1.8W8 times that at 2θ=10°, and a metal component selected from Groups Vb, VIb, VIII and Ib of the periodic table (the content of said pseudoboehmite, calculated as Al2O3, is 10W 80wt%, based on the weight of catalyst.