Abstract: The phase transition behaviors of poly(N,N-diethylacrylamide)(PDEA) and P(N,N-diethylacrylamideco-N-hydroxymethylacrylamide) (P(DEA-co-NHMAA)) aqueous solutions were studied by UV-Vis spectrophotometer. The results suggest that when the mass fraction of NHMAA is below 5.25%, the lower critical solution temperature (TLCS) decreases with the increasing of mass fraction of NHMAA. In contrast, TLCS increases with the increasing of mass fraction of NHMAA when it is higher than 5.25%. Due to the stronger hydrogen bonds between side chains of the copolymer and water, as well as between two side chains of the copolymer in aqueous solution, the phase transition of P(DEA-co-NHMAA) aqueous solution, compared with the phase transition of PDEA aqueous solution, shows less dependence on the concentration. For PDEA and P(DEA-co-NHMAA) aqueous solutions, the abrupt transition from coil to globular and the gradual transition from globular to coil may involve different mechanisms. A model has been proposed for the structure of P(DEA-co-NHMAA) formed in dilute aqueous solution.