Vapor phase isomerization of ethylbenzene to xylene has been studied on various ion-exchanged Y-zeolites. For all catalysts, Pt and BF₃ were impregnated as main components on Y-zeolites and some transition metal ions were exchanged instead of Na⁺ within zeolites. Fe⁺⁺⁺, Cr⁺⁺⁺, Ni⁺⁺, Co⁺⁺, and Ag⁺ were used as transition metal ions. Both selectivity and activity of each catalyst have been investigated. The reaction temperature, reaction pressure, H₂/Ethylbenzene mole ratio, and LWHSV used in the study were 420 ℃, 18 atm, 5.9, 1.62 gr-feed/hr·gr-cat, respectively. The products were analyzed by gas chromatography, and the main products were xylene, benzene, and toluene. It has been found that HX-zeolite has high selectivity and HY-zeolite has high activity, whereas H-mordenite shows both low activity and selectivity. It is also observed that zeolite of HCrY and HFeY show a trend to enhance both activity and selectivity to xylene due to electrostatic potential, whereas zeolities of HAgY and HCoY have low activity and low selectivity to xylene. As the extent of ion exchange increases, it is found that deactivation of catalyst diminishes. The yield of 21 % for isomerization was obtained using HCrY zeolite.