We report the reversible photoinduced self-assembly of a diblock copolymer in a typical hydrophobic ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([C 4mim]PF 6). A series of random copolymers consisting of 4-phenylazophenyl methacrylate and N- isopropylacrylamide (P(AzoMA-r-NIPAm)) show both photoresponsive solubility changes and upper critical solution temperature (UCST) phase transitions in [C 4mim]PF 6. The UCST phase transition temperature of the random copolymer depends strongly on the composition of AzoMA as well as on the photoisomerization state of the azobenzene moiety. The phase transition temperature of P(trans-AzoMA-r-NIPAm) in the dark is lower than that of P(cis-AzoMA-r-NIPAm) under UV-light irradiation. Reversible solubility changes by changing the wavelength of incident light were demonstrated by using the difference between the phase transition temperatures. On the basis of the results, we apply the thermo- and photoresponsive property of the random copolymer to a block copolymer system. A well-defined diblock copolymer (PEO-b-P(AzoMA-r-NIPAm)) was successfully prepared by combining anionic ring-opening polymerization of ethylene oxide (EO) and reversible addition-fragmentation chain transfer (RAFT) polymerization of AzoMA and NIPAm. A PEO-b-P(AzoMA-r-NIPAm) diblock copolymer is found to exhibit low-temperature micelle and high-temperature unimer (upper critical micellization temperature (UCMT)) transition in [C 4mim]PF 6 from dynamic light scattering (DLS) measurements. The aggregation temperature of the diblock copolymer depends on the photoisomerization state of azobenzene, as expected. We demonstrated photoinduced self-assembly of the diblock copolymer in [C 4mim]PF 6 at a "bistable" temperature. Reversibility of the micelle formation and dissolution into single polymer chains was also demonstrated.