The detailed transformation mechanism of methacrolein with methanol to methyl methacrylate over Pd was investigated by density functional theory (OFT) calculation as an attempt to gain more insight into the oxidative esterification reaction. The entire catalytic cycle and reaction energies are computed for various possible paths over Pd20 cluster. It is found that the hydrogen elimination of MAL following acyl group reaction with methoxyl appears to be an energetically favorable pathway in catalyst cycle. The formation of unsaturated alcohol by H addition, and that of saturated aldehyde through the C=C addition by methoxyl and H are also competitive. The formation of hemiacetal intermediate is not necessary for the production of MMA. (C) 2012 Elsevier B.V. All rights reserved.