A novel optical transition for a hydrogel of polyacrylamide and poly( acrylic acid) random copolymer was observed. The poly( acrylamide- co-acrylic acid) hydrogel is opaque white at low temperature, while it will become transparent gradually or sharply on a fast or slow increase of temperature. Meanwhile, the hydrogel can swell or collapse with changing temperature. Both optical transition and volume swelling are reversible responding to temperature stimuli. The FT- IR, laser Raman, and 1H NMR spectroscopy demonstrated the driving force of optical and volume transition: formation and dissociation of hydrogen bonds in the hydrogel. The heterogeneity of the network results in these reversible transition responding to temperature stimuli. The effects of monomer, initiator, and cross- linker on the optical transition of the hydrogel were studied in detail. The formation and dissociation of hydrogen bonds in the limited domain of the hydrogel is related to the network structure. Because of the property of swelling or collapse, the P( AAm-co- AAc) hydrogel has potential value of application in drug controlled release.