Behavior of catalytic cracking reactions of particle cluster in fluid catalytic cracking (FCC) riser reactors was numerically analyzed using a four-lump mathematical model. Effects of the cluster porosity, inlet gas velocity and temperature, and coke deposition on cracking reactions of the cluster were investigated. Distributions of temperature, gases, and gasoline from both catalyst particle cluster and an isolated catalyst particle are presented. The reaction rates from vacuum gas oil (VGO) to gasoline, gas and coke of individual particle in the cluster are higher than those of the isolated particle, but it reverses for the reaction rates from gasoline to gas and coke. Less gasoline is produced by particle clustering. Simulated results show that the produced mass fluxes of gas and gasoline increase with the operating temperature and molar concentration of VGO, and decrease due to the formation of coke.