Simulation of diffusion phenomena of aromatic hydrocarbons in the channel of three different zeolites (ZSM-5, mordenite, and Y-type zeolites) was carried out by using a commercial CAMD (computer-aided molecular design) software. The diffusion phenomena of model aromatic hydrocarbons, such as benzene, naphthalene, phenanthrene, and pyrene, were found to depend greatly on molecular sizes of both the channel of these zeolites and model compounds. The relationship between such diffusion phenomena in the zeolites and product distribution of these aromatic hydrocarbons in hydrocracking reaction is briefly discussed.