The modification of the Ag - MoO3/ZrO2 catalyst with metallic chloride was studied for a gas-phase epoxidation of propylene by molecular oxygen. As a modi. er, the presence of NaCl, CsCl or CaCl2 can effectively regulate an electronic property of the Ag MoO3/ ZrO2 catalyst and improve obviously its catalytic epoxidation performance. When NaCl (1000 ppm), CsCl (1500 ppm) or CaCl2 (1000 ppm) was used to modify the 20% Ag - 4% MoO3/ZrO2 catalyst, the selectivity to propylene oxide (PO) increased to 63.5%, 66.7% and 62.9%, from 47.9%, respectively, and the corresponding O-2 conversion was 7.3%, 7.7% and 8.1%, respectively. The studies of XPS indicates that the electron-interaction between the active component of catalyst and metallic chloride has occurred obviously, and the presence of chloride can enhance the electron-deficient property of Ag to improve the electrophilic property of adsorbed oxygen and inhibit an isomerization and deep oxidation of PO to improve the selectivity to PO.