Modified Ag/α-Al2O3 catalysts were prepared for the gas phase epoxidation of propylene by molecular oxygen. We used temperature-programmed desorption of oxygen (O2-TPD) to study the desorption behavior of oxygen from the catalyst's surface. Results showed that over the Ag/α-Al2O3 catalyst propylene was completely oxidized to CO2 and H2O. When the catalyst was modified with K2O, a small amount of propylene oxide (PO) was obtained. When the catalyst was modified with Y2O3, a very small amount of propanal and acetone was obtained. Adding 0.1% (w) Y2O3 to the Ag-K2O/α-Al2O3 catalyst significantly improved its catalytic performance for the gas phase epoxidation of propylene. Under reaction conditions of 0.1 MPa, 245 ℃, a feed gas of 20%C3H6/8%O2/72%N2 and gas hourly space velocity (GHSV) of 2000 h-1, the conversion of propylene was 4.0%and the selectivity to PO was 46.8%over the 20% (w)Ag-0.1%Y2O3-0.1%K2O/α-Al2O3 catalyst. O2-TPD showed that when the 20%Ag/α-Al2O3 catalyst was modified with Y2O3, K2O or Y2O3-K2O, the amount of adsorbed oxygen in the higher temperature region for the total oxidation of propylene decreased and the amount of adsorbed oxygen in the lower temperature region for the gas phase epoxidation of propylene was unchanged, which improves the selectivity to PO.