Abstract: A process of thermally cracking a heavy petroleum oil wherein the heavy petroleum oil is treated successively in a cracking furnace and then in a perfect mixing type tank reactor. The thermal cracking in the cracking furnace is performed at a temperature at the outlet of the cracking furnace of 450.degree.-520.degree. C. with a conversion of at least 60-75% of the overall conversion rate while the thermal cracking in the tank reactor is performed at a temperature of 400.degree.-450.degree. C. a pressure of from ambient pressure to 1 kg/cm.sup.2 for a period of time of less than 30 minutes but not less than 10 minutes while feeding steam having a temperature of 435.degree.-700.degree. C. to the tank reactor in an amount of 8-20% by weight of the heavy petroleum oil fed to the cracking furnace.