The deuterium isotope effects on the volume phase transition of a typical temperature-sensitive polymer gel (poly(N-isopropylacrylamide) (PNIPAM) gel) and the phase separation of the linear polymer (PNIPAM) have been investigated. For the comparison between the bulk change and the microenvironment change, the deuterium isotope effects of PNIPAM gel and linear PNIPAM solution have also been investigated by using a fluorescence probe. Both the transition temperature of the polymer gel and the phase separation temperature of the linear polymer in heavy water are about 0.7 degrees C higher than those in water. However, the deuterium isotope effects on the difference of the transition temperatures in the heating process and the cooling process in the microenvironments of PNIPAM gel and linear PNIPAM solution are not observed.