In the fluid catalytic cracking reactor heavy gas oil is cracked into more valuable lighter hydrocarbon products. The reactor input is a mixture of hydrocarbons which makes the reaction kinetics very complicated due to the involved reactions. In this paper, a four‐lump model is proposed to describe the process. This model is different from others mainly in that the deposition rate of coke on catalyst can be predicted from gas oil conversion and isolated from the C1–C4 gas yield. This is important since coke supplies heat required for endothermic reactions occurring in the reactor. By this model we can also conclude that the C1–C4 gas yield increases with increasing reactor temperature, while production of gasoline and coke decreases.