The mechanism of xylene isomerization over HY-zeolite and Pt-loaded HY-zeolite （Pt/HY） was studied. And the isomerization of mixed xylene containing ethylbenzene over the above catalysts was also investigated. As the results of experiments, vapor-phase isomerization of xylene over HY-zeolite proceeded via intermolecular transalkylation mechanism and intramolecular 1,2-methyl shift mechanism simultaneously, and the contribution of each mechanism to the overall reaction depended on reaction condition. Over Pt/HY, the isomerization of xylene occurred through hydrogenated intermediates and had tendency to proceed via the route involving more stable carbocation preferentially. In the isomerization of mixed xylene over HY-zeolite, the extent of transalkylation of ethylbenzene was higher than that of xylene.