Propylene epoxidation is investigated on RuO2(110) and oxygen added RuO2-O-ot(110) surfaces by periodic DFT computational method. The desired product propylene oxide (PO) as well as the undesired products acetone (AC) or propionaldehyde (PA) form on both surfaces through either surface intermediate oxometallopropylene (OMMP) or direct oxygen insertion mechanisms. On RuO2(110) surface, nucleophilic lattice oxygen at bridge position (O-br) favors the stable surface intermediate mechanism where high energy requirements for forward reactions are demonstrated in our calculations. On RuO2-O-ot(110) surface, however, higher reactivity of the electrophilic oxygen (Oot) species lowers the reaction barriers and enables an exothermic reaction path to the direct oxygen insertion for PO production. Therefore, RuO2-Oot surface is expected to show a higher PO rate. (C) 2016 Published by Elsevier B.V.