A thermal cracking process for producing petrochemical products from hydrocarbons which comprises the steps of: burning hydrocarbons with oxygen in the presence of steam to produce a hot gas of from 1300.degree. to 3000.degree. C. comprising steam; feeding a mixture of methane and hydrogen to the hot gas in such a way that a methane/hydrogen molar ratio is over 0.05; further feeding starting hydrocarbons to the hot gas comprising the methane, hydrogen and steam so that the starting hydrocarbons containing hydrocarbon components of higher boiling points are, respectively, fed to higher temperature zones; subjecting the starting hydrocarbons to thermal cracking while keeping the cracking temperature at 650.degree. to 1500.degree. C., the total residence time at 5 to 1000 milliseconds, the pressure at 2 to 100 bars, and the partial pressure of hydrogen, after thermal cracking of a hydrocarbon comprising hydrocarbon components whose boiling point exceeds 200.degree. C., at least 0.1 bar; and quenching the resulting reaction product.