PURPOSE: Heavy hydrocarbon oil is subjected to catalytic hydrogentation treatment in the presence of a catalyst and the resultant liquid component is pyrolyzed in the presence of steam to inhibit the formation of heavy fuel as a by- product and produce olefins and monocyclic aromatic hydrocarbons in high yield.

CONSTITUTION: Asphalten is removed from the bottoms after atmospheric distillation of crude oil through the solvent deasphalting process and the resultant heavy hydrocarbon oil is subjected to catalytic hydrogenation treatment in the presence of a catalyst of nickel or molybdenum supported on an inorganic carrier at 300W450°C and 60W200kg/cm2 and separated into the gas fraction mainly containing hydrogen and the liquid fraction. Then, the liquid fraction is pyrolyzed with steam at a steam to hydrocarbon oil weight ratio of 0.5W1.2, 750W850°C temperature and 0.1W1.0sec. residence time to produce olefins and monocyclic aromatic hydrocarbons.