Epoxidation of C3H 6 to C3H6O by air was studied over a silver catalyst modified with alkali or alkaline earth chloride salts. The catalyst preparation factors and the operational conditions could affect obviously the catalytic epoxidation property of the silver catalyst. It was shown that, as a promoter of the silver catalyst, NaCl or BaCl2 is more suitable than LiCl or NH4Cl. The loading of NaCl should be controlled at about 3.8 wt%. Using a feed gas of 10% C3H6/air at a space velocity of 1.75 x 10(4) h(-1), 18.6% C3H6 conversion and 33.4% selectivity to C3H6O were obtained at 350 degrees C. Using a feed gas of 5% C3H6/air at a space velocity of 2.4 x 10(4) h(-1), 54.0% C3H 6 conversion and 26.3% selectivity to C3H6O were obtained at 390 degrees C.