A novel amphiphilic diblock copolymer composed of a hydrophilic poly(ethylene oxide) block and a hydrophobic block copolymerized by azobenzene-containing methacrylate and N-isopropylacrylamide was synthesized using ATRP. The polymer micelles showed dual responsiveness to heat and light. The size of the micelles was dependent on temperature and the encapsulated substance in the hydrophobic cores was released during heating and cooling processes. The hydrophobicity of the micellar cores appeared as a reversible change in response to light with neither disruption of the micelles nor leakage of the encapsulated substance while H-aggregation of the azobenzene moieties was detected.