Epoxidation of propylene to propylene oxide by molecular oxygen was studied over a modified Ag-MoO3 catalyst. The results show that MoO3 plays an important role in improving the efficiency of the catalyst, and a suitable content of MoO3 is 40-50 wt%. XPS reveals that some of the silver and molybdenum in the catalyst exist as Ag+ and Mo(6-delta)+, respectively. The promotion effect of NaCl, Ce(NO3)(3), BaCl2 and CsNO3 on the Ag-MoO3 catalyst was studied. As a modifier of the Ag-MoO3 catalyst, NaCl or Ce(NO3)(3) are more suitable than BaCl2 or CsNO3 and the optimal loading of NaCl or Ce(NO3) is about 2wt%. Using a feedstock gas of 15.6% C3H6, 12.2% O-2 and balance N-2 without any addition of NO, EtCl or CO2 at a space velocity of 4500 h(-1), 6.8% O-2 conversion and 53.1% selectivity to propylene oxide were achieved over the Ag-MoO3-2.0% NaCl catalyst at 400 degreesC. At 450 degreesC the O-2 conversion and selectivity to propylene oxide were 11.4 and 43.6%, respectively.