Poly(N-isopropylacrylamide) prepared by free radical polymerization was successfully fractionated into several fractions by using a carefully water-freed acetone as solvent and n-hexane as non-solvent. Based on the viscometric and osmometric studies of these fractions, the intrinsic viscosity-molecular weight relationships were determined. It was also found that the aqueous solution exhibited an abnormal concentration dependence of viscosity at temperatures higher than 25 degree C and the phase separation at 30 degree C.