Three-dimensional computational fluid dynamics simulations were performed to study gas flow, heat transfer, and catalytic reactions of vacuum gas oil inside spherical and ellipsoidal clusters consisting of fluid catalytic cracking catalyst particles. Simulation results with respect to gas flow inside the cluster and convective heat transfer between the gas and catalyst particle agreed well with results from the literature. Simulation results indicated that the gas stayed longer and had a higher temperature inside the cluster than that in the main stream, which intensified secondary reactions of gasoline species in the gas. At a higher cluster porosity, it was predicted that the gas could flow through the cluster with lower resistance and that secondary reactions lessened.