The transition behavior of swelling and deswelling of water-soluble poly(N-isopropylacrylamide) was calculated by a decorated UNIQUAC model with blob concept. The deswelling behavior of hydrogels at high temperatures was also reported with shrinking volumes during the transition. Polymer chains were scaled within a directly interacting size as blobs, which are composed of a segmentalized polymer chain and solvent molecules and counted blobs of hypothetical components or segments. The interaction parameters of bulks, UNIQUAC interaction parameters, were approximated with temperature-independent site interaction energies following the blob renormalization transformation. The swelling gel transition volume at each given temperature was calculated with the decorated UNIQUAC model. The blob rescaling model successfully described the first order transition of swelling-deswelling behaviors.