Abstract: According to the mechanism of alkylation of toluene and the mechanism of isomerization of meta-xylene, which were suggested by Kaeding and Guisnet seperately, CNDO/2 method has been used to calculate the total energy of complexes in the pore of HZSM-5, Co/ZSM-5 and Ni/ZSM-5. The result can be used to explain the increase selectivity of para-xylene when the ZSM-5 modified with Co and Ni was used as catalyst. We also describe the functions of the pore of ZSM-5 and the metal atom in the pore by analyzing the diatomic interaction energy.