Styrene epoxidation with hydrogen peroxide catalyzed by Mn(III) complex was studied using DFT calculation method. The previous mechanisms of metal complex-catalyzed epoxidation were found unsuitable for the Mn(III) complex used in this work. Reasonable mechanisms were proposed and investigated. Lowering of barrier was found in catalyzed reaction. The side reaction, oxidation of styrene, was also investigated. The Gibbs energy changes of epoxidation and oxidation were calculated to give a general prediction of product distribution. (C) 2010 Elsevier B. V. All rights reserved.