Abstract: The pretreated tannin was used to modify the UF resin in this study. The different adding stages and amounts of tannin were studied, the DSC and the ¹³C-NRM were used to analyze the structure of the resins. The results showed that adding 7% of tannin at the acid stage to modify the UF resin had the best effect. By this way, the free formaldehyde content of the resin reduced 58%. The 24 h absorbing water thickness expansion rate of particle board made by this kind resin was 11.6%, it reduced 37% compared with unmodified ones. The addition reaction and polycondensation reaction were hindered by the tannin, so if the tannin was added too much, the free formaldehyde content of the modified resin would be higher than the unmodified ones.