Ag/BaCO3 catalysts with different Ag crystallite sizes, prepared by the reduction-deposition method, were developed for gas-phase epoxidation of propylene to propylene oxide (PO) by molecular oxygen. Effects of preparation conditions, such as pretreatment of organic amines of BaCO3 support, pretreatment way of ethylene diamine, reduction temperature of HCHO and calcination temperature, on the catalytic performance and Ag crystallite size of Ag/BaCO3 catalyst were investigated. Ag/BaCO3* catalyst, with pretreatment of BaCO3 support by ethylene diamine and prepared at the reduction temperature of HCHO of 10 degrees C and the calcination temperature of 250 degrees C, exhibited better catalytic performance and good durability, in which 12.5% of propylene conversion and 36.9% of PO selectivity were achieved under the reaction conditions of 20%C-3 H-6-10%O-2-70%N-2, 200 degrees C, 0.1 MPa and GHSV of 3000 h(-1). Both lower reduction temperature of HCHO and lower calcination temperature can get smaller Ag crystallite sizes, which is more effective for epoxidation of propylene over Ag/BaCO3* catalyst. The catalytic reaction mechanism of Ag/BaCO3* catalyst for epoxidation of propylene is that propylene in the gas phase reacts with molecular oxygen species adsorbed on the catalyst surface to produce PO and follows Rideal-Eley mechanism. (C) 2016 Elsevier B.V. All rights reserved.