Abstract: An object is to increase cracking rate of heavy fraction oils while producing a lessened amount of dry gases generated by the hydrogen transfer reaction and by the overcracking to obtain light fraction olefins in a high yield. A process for the fluid catalytic cracking of heavy fraction oils, which comprises steps of feeding the heavy fraction oils to a raw oil introducing portion provided at a reaction zone inlet; feeding a part of a regenerated catalyst taken out of a catalyst-regenerating zone to a catalyst introducing portion provided at a reaction zone inlet; and feeding another part of the regenerated catalyst taken out of the catalyst-regenerating zone to at least one catalyst introducing portion which is provided between the catalyst introducing portion provided at the reaction zone inlet and reaction zone outlet, the catalytic cracking in the reaction zone being carried out under conditions of a contact time of 0.1 to 3.0 sec. a reaction zone outlet temperature of 530 to 700.degree. C.