Abstract: Structure formation of UF resins under strong and weak acidic conditions were investigated with high resolution ¹³CNMR (DRX-500).The structures of resins prepared under acidic conditions was difference greatly with typical UF resin.The crucial factor seems to be the different reaction rate of methlolation and methylene bond formation.Under strong acidic condition,methylene bond formation is fast than methlolation,so the main constituents of UF resins are contents with much high methylene.Strong acid catalyzed UF resin characters as Uron and high cross linkages contain.Under weakly acidic conditions,since methlolation and methylene bond formation go with almost equivalent rate,the structure constituents of resin seem to be average of typical resin and strong acidic catalyzed resin.