This paper concerns the prevention of oxidative damage in commercial RO/NF membranes by treatment with tannic acid. We investigated the effects of periodic tannic acid treatments on separation performance and flux in membranes With oxidative damage. First, we Studied LES90 as an NF membrane. We found that the untreated membrane showed oxidative damage with the increase in flux and decrease in separation performance. Furthermore, the treated membrane showed tolerance to oxidants as stable flux and stable separation performance. We assumed that tannic acid was more likely to react with oxidants than the membrane. Specifically, tanmic acid reacted with the oxidants before the membrane. In this way, tannic acid functioned to protect the membrane from oxidative damage. Next, we studied ES10 as an RO membrane. We found that the treated membrane,showed tolerance to oxidants. However, the treatment was less effective than in the case of LES90. We supposed that the adsorption level of tannic acid in ES10 was less than in LES90.