Stimuli-responsive diblock copolymers with a thermosensitive segment and a hydrophilic segment have been synthesized via sequential living cationic copolymerization, which involves a poly(vinyl ether) with oxyethylene pendants exhibiting LCST-type phase separation in water and a poly(hydroxyethyl vinyl ether) segment. Highly sensitive and reversible thermally induced micelle formation and/or physical gelation were observed with such diblock copolymers. For example, the flow behavior of an aqueous solution of a diblock copolymer varied from Newtonian to non-Newtonian and plastic flow within a very narrow temperature range. TEM and SANS studies showed that the observed change in viscosity was due to the formation of a macrolattice with body-centered-cubic symmetry of spherical micelles in aqueous solution. The critical temperature of micelle formation and/or physical gelation could be varied by altering the combination of two segments in the diblock copolymer. On the basis of these results, several systems incorporating various patterns of physical gelation behavior have been developed, and a strategy for constructing stimuli-responsive systems with block copolymers was established.