The direct gas-phase oxidation of propylene with molecular oxygen was carried out over Ti-MCM-41 impregnated with metal nitrates to produce propylene oxide (PO) selectively. The effects of Ti-loading on MCM-41, nitrate additives, acid treatment of the support, and the presence of molecular oxygen were investigated. The PO yield increased with increasing Ti content of Ti-MCM-41 and reached a maximum at an optimum Si/Ti ratio of 100. Calcium nitrate was the most suitable additive among various nitrates of alkali and alkaline earth metals investigated, and the acid treatment of Ti-MCM-41 effectively improved its catalytic performance for the PO formation. The presence of both nitrate and molecular oxygen was essential for the PO formation. A synergetic mechanism between the Ti species and metal nitrate was proposed to be responsible for the selective fort-nation of PO.