The fine control of the pore-opening size of HM zeolite by chemical vapor deposition of Si(OCH_3)_4 has been investigated. Si(OCH_3)_4 can be deposited by reaction with the hydroxyl groups on the zeolie. After calcination in oxygen silica is deposited on the externai surface of the zeolite and produces silica coated HM zeolite (SiHM). The SiHM samples with different amount of silica deposited have been characterized by adsorption experiments and temperature-programmed desorption of NH_3. It has been found that the deposition of silica does not change the acidity of the zeolite internal surface but reduces the pore-opening size. For toluene disproportionation reaction the para-selectivity of the SiHM zeolite catalysts is increased markedly. Under the same reaction conversion the mole fraction p-xylene in the xylene mixture can be raised up to above 0.40 when using SiHM as the reaction catalyst instead of the original HM zeolite. For isomerization of o-xylene the improvement in para-selectivity has not been observed, because the reaction is not mainly controlled by the diffusion of the reaction products.