The roles of promoter in HCrY-zeolite catalysts have been studied in the vapor phase isomerization of ethylbenzene to xylene. As a promoter, one of alkali metals（lithium, sodium and potassium） or one of alkaline earth metals(magnesium, calcium and strontium) was added to an HCrY-zeolite catalyst containing Pt 0.5 wt% and BF₃ 1.07 wt% in order to increase the catalytic activity and to decrease the rate of coke deposition which causes catalytic deactivation. The experiments were carried out at 420 ℃ of reaction temperature, 18 atm of reaction pressure, 5.9 of H₂/ethylbenzene mole ratio, and 1.62 gr-feed/hr·gr-cat of LWHSV(Liquid Weight Hourly Space Velocity). For the magnesium-promoted HCrY-zeolite catalysts, total conversion exhibited a maximum and selectivity for cracking a minimum. The yield of 27 % for isomerization was obtained using HCrMgY-6 mole%Mg-zeolite. For the HCrMgY-zeolite, the best catalytic activity was shown with the ion-exchanged order of Cr and then Mg, but not with Mg and then Cr nor simultaneous exchange. The results were explained in terms of the change in catalytic acidity due to the change of polarizing power caused by the addition of promoters.