The catalytic performance and availability of heteropoly compounds in vapor phase isomerization of ethylbenzene to xylene have been studied. 12-molybdophosphoric acid and its salts of four first row transition metals（Cr, Fe, Co and Ni） were chosen as heteropoly compounds, and they were supported together with platinum over Y-zeolite. The experiments were carried out at 400℃, 18 kg/cm² of H₂ pressure, 4.6 of H₂/ethylbenzene mole ratio and 1.5 gr-feed/hr·gr-cat of LWHSV (Liquid Weight Hourly Space Velocity). As the results of experiments, heteropoly acid enhances the activity and selectivity of isomerization and the activity of isomerization is strongly affected by the method of catalyst preparation. In case of 12-molybdophoshates, their acidities vary with the kind of countercations and it seems that the acidity is closely related to the electro-negativity of the corresponding countercation. The pretreatment with hydrogen shows an echanced activity and selectivity.