In order to develop photoresponsive polymeric micellar systems, an amphiphilic block copolymer consisting of anthracene moieties was successfully prepared by atom transfer radical polymerization. In an aqueous solution, the photodimerization of the anthracene moieties occurred in the micellar core upon irradiation (lambda > 360 nm), keeping the mean diameter constant. The photodimerization caused the interpolymer reaction to form a core-crosslinked micelle.