Abstract: The effect of CaOmodification on the selectivity of ZSM-5 was studied. The selectivity of the modified catalyst toward dimctyl benzene in the aromatzation of propenc and the methylation of tolune was increased by a factor of 2.7 and 1.6 respectively in relation to the unmodified one.The calculation based on the window model was made for the charge density of the carbon atom in tolune when on the modified and unmodified catalyst.The results show that the charge density for the para-carbon atom of tolune is largely increased in the CaOmodified ZSM-5 and the energy in I₃ position is the lowest,thus favor the electrophilic reactions.This explains the improved selectivity toward dimethyl benzene of ZSM-5 upon CaOmodification.