Abstract: The Ag-MoO3 catalyst for direct gas phase epoxidation of propylene was prepared, and in situ FT-IR technology was used to study the adsorption and reaction behavior of propylene, propylene oxide (PO), and the gaseous mixture of propylene and oxygen on the surfaces of Ag and Ag-MoO3 catalysts. The results indicate that on using single Ag catalyst the selectivity to PO is very low, and Ag-MoO3 is an efficient catalyst for the direct epoxidation of propylene using molecular oxygen as oxidant. The studies of in situ FT-IR show that no chemical reaction occurs, within the range of the experimental temperatures, when propylene is adsorbed on the surface of Ag or Ag-MoO3 catalysts, whereas, the open-ring, polymerization, and coking of PO adsorbed on the Ag or Ag-MoO3 catalysts can occur at higher temperature. The presence of MoO3 in the Ag-MoO3 catalyst can inhibit the open-ring and deep reactions of PO to some extent, despite decreasing its activity, which leads to an increase in the selectivity of PO. The lower reaction temperature and shorter contact time are favorable to inhibit the deep reaction of PO on the surface of the catalyst.