The behavior of catalytic cracking reactions of particle cluster in fluid catalytic cracking risers was numerically analyzed using a four-lump mathematical model. The effects of the cluster porosity, inlet gas velocity, and cluster formation on cracking reactions were investigated. Distributions of temperature, gases, and gasoline from both the catalyst particle cluster and an isolated catalyst particle are presented. Simulated results show that the reactions from vacuum gas oil (VGO) to gasoline, gas, and coke of individual particle in the cluster are slower than those of the isolated particle, but faster for the reaction from gasoline to gas and coke. Particle clustering will reduce the reaction rates from VGO to gasoline, gas, and coke and increase the reaction rates from gasoline to gas and coke. Less gasoline is produced by particle clustering. More gas and gasoline are produced for the downward moving cluster than for the upward moving cluster. The cluster formation decreases the reaction rates and reduces gas and gasoline production.