PURPOSE: To prepare a gaseous olefin and a monocyclic aromatic hydrocarbon economically with high yields, by removing high boiling substance from a product of thermal cracking heavy hydrocarbon oil in the presence of hydrogen, hydrogenating the product and performing thermal cracking with steam.

CONSTITUTION: A coking inhibitor consisting of fine particles which contain typical element compd. and transition metal (or compd. which contains it) is added to a heavy hydrocarbon oil and the oil is thermally cracked at 380W550°C under the pressure of 30W300kg/cm2 for 2W120min. High-boiling substances are removed from the product of thermal cracking and the product is hydrogenated at 250W500°C under the pressure of 30W300kg/cm2 at a liquid space velocity of 0.05W5.0hr-1 and at a hydrogen/oil ratio of 100W2,000Nl/l. Hydrogen-containing gaseous product is removed from the hydrogenation product and liquid product is thermally cracked at 700W900°C at a dwell time of 0.05W 2.0sec and a steam/liquid product ratio of 0.2W2.0 by weight and gaseous olefins and monocyclic aromatic hydrocarbons are recovered from the product.