Hydrocracking of phenanthrene was conducted in a batch autoclave system using ZSM-5 (5.5 Angstrom), mordenite (7 Angstrom) and Y-type zeolite (8 Angstrom) as catalysts. The number in brackets is the size of the pore of each zeolite. Close examination of product distribution derived from hydrocracking of phenanthrene indicated that it is strongly affected by the pore size of the zeolite used. Based on the computer aided molecular design studies of each zeolite, the way of aromatic compounds accessing towards the entrance of the pore of zeolite was visualized, herewith being able to explain the characteristic distribution of the products.