Abstract: A process and apparatus capable of cracking hydrocarbon to produce a reaction product containing a high proportion of ethylene. A hydrocarbon such as naphtha is vaporized and admixed with superheated steam at high temperature in a mixing device. The resulting hydrocarbon-steam mixture is passed through a reaction zone consisting of a reactor conduit which extends through a passageway defined in a radiation block structure. Heating gases at extremely high temperatures are directed through the passageway, co-currently with the hydrocarbon-steam mixture, to produce a desirable heat flux for the cracking reaction. A short residence time in the reactor conduit is maintained to prevent undesirable side reactions.