We found that a simple K+-modified CUOx-SiO2 catalyst prepared by a sol-gel method was efficient for the epoxidation of propylene by oxygen. The catalyst did not require prereduction, and propylene oxide (PO) could be formed with a selectivity of 78% under oxygen-rich conditions. We demonstrated that Cu(I) formed during the reaction over the K+-modified CuOx-SiO2 catalyst was the active site for propylene epoxidation by oxygen. Cu(I) was proposed to account for the activation of oxygen, generating an active oxygen species for propylene epoxidation, while K(+)enhanced the PO selectivity probably by neutralizing the Lewis acid sites on catalyst surface. The enhanced dispersion of copper species due to the interaction with K+ may also contribute to the increase in PO selectivity.