Water-soluble and thermosensitive 4VPNBz-NIPAAm terpolymers were obtained by the reaction of 4-vinylpiridine(4VP)-N-isoprppylacrylamide(NIPAAm) copolymers with benzylchloride at 100 or 120°C for 1-24h in dimethylformamide(DMF). The quaternarization of piridine moiety at 100°C increased with increasing time, but at 120°C the quaternarization of piridine moiety increased up to 1h, then decreased with increasing time. The 4VPNBz-NIPAAm terpolymers in aqueous solution were water-soluble below 32°C, but insoluble above 32°C. In particular, the 4VPNBz-NIPAAm terpolymers in NaCl aqueous solution more than 0.1mol·dm-3 precipitated above 34°C. The solubility behavior of 4VP-NIPAAm copolymer in DMF/H2O, DMSO/H2O, and methanol/H2O mixture solution was investigated. The 4VPNBz-NIPAAm terpolymers exhibited flocculation ability against E.coli suspension.