Abstract: Propylene oxide was synthesized by oxidizing propylene catalyzed by transition metal salts with molecular oxygen in the presence of acet aldehyde under mild condition.This method was safer for the peroxide concentration was maintained at an extremely low level and was convenient and of high yield,as compared with the traditional oxidation methods.The influence of several factors on the reaction was investigated such as the type of catalyst, the ratio of reactants，etc.It was found that when using nickel salt as the catalyst, setting the ratio of acetaldehyde and propylene as 1∶1, reacting 90 min under 70℃,the yield of propylene oxide might reach about 90%.