The secondary reaction of ethylene oxide over silver catalyst supported on MCM-41 mesoporous material was investigated by the in-situ IR method. MCM-41 mesoporous material with a large content of hydroxyl group was very active in catalyzing the isomerization of ethylene oxide to acetaldehyde. The reduction of hydroxyl group by silane treatment or silver loading suppressed the isomerization of ethylene oxide. The selectivity for ethylene oxide in the partial oxidation of ethylene is enhanced by silane modification revealing the acceleration of the consumption of ethylene oxide on the hydroxyl group.