Abstract: A process for producing olefins by steam cracking of a heavier hydrocarbon includes the steps of: subjecting a heavier hydrocarbon to a first separation to obtain a first vapour stream and a first liquid stream, partially condensing (second separation) the first vapour stream to obtain a condensate (third heavier fraction), introducing at least a portion of the third heavier fraction into the first liquid stream to obtain a first modified liquid stream, subjecting the first modified liquid stream to a third separation to obtain a second vapour stream and a second liquid stream, and carrying out steam cracking on the second vapour stream to obtain a cracked gas effluent containing the olefins. This process inhibits the coking of the convection zone, and provide a high utilization rate of heavier hydrocarbon (crude oil) and a high ethylene yield.