This invention relates to a process for cracking hydrocarbon oils. The process comprises contacting a hydrocarbon oil with a catalyst that has been contacted with an atmosphere containing a reducing gas, separating cracked products and the catalyst, and regenerating the catalyst. The catalyst is a cracking catalyst containing a metal component, or a catalyst mixture of a cracking catalyst containing a metal component and a cracking catalyst free of metal component. The catalyst is contacted with the atmosphere containing a reducing gas at a temperature of 100 to 900.degree. C. for at least 1 second, and the amount of the atmosphere containing a reducing gas is not less than 0.03 cubic meters of reducing gas per ton of the cracking catalyst containing a metal component per minute, at a pressure of 0.1-0.5 MPa in the reduction reactor. The process has enhanced capability for desulfurizing and cracking heavy oils.