This paper concerns improving quality standards of commercial NF membranes by use of tannic acid. Secondly, it considers the way in which tannic acid can reduce oxidative damage in RO membranes. We investigated its effects on separation performance and flux in NF membranes. We found that Chinese gallnut had the highest effect on the separation performance of LES90. Next, we studied the impact of treatment mode, concentration and time in regard to this effect. We found that permeation treatment with pressure was superior to submersion treatment. To improve separation performance, treatment with more than 5 mg/L of tannic acid for 10 minutes was required. Furthermore, this treatment was able to reduce three kinds of oxidative damage in RO membranes, Thus, treatment with tannic acid improved the performance of the commercial RO/NF membranes.