Stable temperature sensitive polymer nanometer gel was prepared by the soap—less emulsion polymerization, which initiated by light and the diameter was successful controlled less than 100nm. The reactive system were composed by the aqueous main monomer N—isopropylacrylamide and methylenebisacrylamide, MBA. The effects of system compose and conditions on the diameter were studied. The result indicated that under the CMC,the gel diameter will continue declining and be invariable finally with increase the emulsifier concentration. Higher poly dispersity and smaller particles can be got in the polymerization initiated by light compared to the therm initiate.