An amphiphilic diblock copolymer, poly(ethylene oxide)-block-poly {6-[4-(4-ethoxyphenylazo) phenoxy] hexyl methacrylate}, was synthesized using atom transfer radical polymerization. In the solution of tetrahydrofuran and water, the diblock copolymer formed micrometer scale vesicles by self-assembly. Under the irradiation of UV light at 365 nm, these microvesicles generated such photoresponsive behavior as photoinduced expansion, swelling and fusion due to trans-cis photoisomerization of azobenzene units. The self-assembled vesicles even enlarged to be about five times their original size because of photoinduced fusion.