The thermoresponsive dispersion properties of triarm-shaped diblock copolymers of thermoresponsive poly(N-isopropylacrylamide) {poly(NIPAAm)} as a core unit and poly(N-hydroxyethylacrylamide) {poly(HEAAm)} as a hydrophilic outer shell were investigated. The core and the outer shell were synthesized by atom-transfer radical polymerization (ATRP). The block polymer was completely dissolved in water up to 10(-6) M with no turbidity even above lower critical solution temperature (LCST). Above the LCST (ca. 33.6 degrees C) polymer aggregates, in which the NIPAAm moiety was shrunk and formed a hydrophobic core, were monitored by dynamic light scattering measurements, while no signal was observed below the LCST. The size of globular aggregates with diameter in the range 35 to 170 nm was kinetically controlled by incubation in the transition-temperature region near the LCST. The size was stable above that temperature region.