Catalytic cracking experiments of vacuum gas oil on fluid catalytic cracking (FCC)-type catalysts are carried out in a fluidized bench-scale batch Chemical Reactor Engineering Centre (CREC) riser simulator reactor. These experiments are conducted under operating conditions similar to those of the industrial FCC process in terms of temperature, catalyst-to-oil ratio, partial pressure of reactant and products, and reaction times. The crystallite size of the supported zeolite is varied between 0.4 and 0.9 microns with both activity and selectivity being monitored. A five-lump kinetic model describing the catalytic cracking of vacuum gas oil is considered, which accounts for diffusional constraints experienced by hydrocarbon species while evolving in the zeolite pore network. This study provides insights into the effect of intracrystalline diffusion in the catalytic cracking of heavy feedstocks. Results show that the catalyst with the smaller crystallite size provides higher activity and selectivity for desirable intermediate products (gasoline) and lower selectivity for terminal products (coke).