HZSM-5 zeolites modified by Si(CH3)3 Cl were studied by TPD, IKand 27Al NMR to characterize their surface properties and bycatalytic reaction of toluene disproportionation.It was found that the concentration of the stronger Bronsted acid sites were decreased and the stronger Lewis acid sites were increased after modifieation with Si(CH3),Cl. The TPD peaks of NH3, presumally attributed to stronger acid sites (peak Ⅱ ) were decreased by the deposition of Si(CH3)3 Cl on the surface of the zeolites.The selectivity to the para xylene can be attained not only by controlling the pore opening and channels of the zeolite, hut also by changing the concentration of the stronger acid sites and the acidic type on the surface of the zeolite. HZSM-5 zeolites modified by Si (CH3) 3 Cl were studied by TPD, IK and 27Al NMR to characterize their surface properties and by catalytic reaction of toluene disproportionation. It was found that the concentration of the stronger Bronsted acid sites were decreased and the stronger Lewis The TPD peaks of NH3, presumably attributed to stronger acid sites (peak II) were decreased by the deposition of Si (CH3) 3 Cl on the surface of the zeolites.The acid sites were increased after modifieation with Si (CH3) selectivity to the para xylene can be attained not only by controlling the pore opening and channels of the zeolite, hut also by changing the concentration of the stronger acid sites and the acidic type on the surface of the zeolite.