The solution properties of poly(N-isopropylacrylamide) (PNIPAM) in water was studied by viscometry. The effects of inter- and intramolecular interaction can be controlled by a different solution preparing process. The results revealed that the change of polymer chain conformation was irreversible when PNIPAM underwent coil-to-globule to globule-to-coil cycle, because the new interaction points formed during the process of chain contraction by hydrogen bond or van der Waals interactions. The interaction point was stable enough in the experimental conditions used here and retarded the full recovery of the molecular conformation.