The well-defined block copolymer of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAm) was synthesized by the reversible addition fragmentation transfer (RAFT) polymerization, and its thermo-induced aggregation process in dilute aqueous solution was studied by laser light scattering. At temperatures below the cloud point of PNIPAm, the PEO-b-PNIPAm chains were associated together to form large and loose structures, which were in a fast equilibrium with the single chains. The association was weakened with increasing concentration, which was against the common ideas about aggregation. Because of such "abnormal" behavior, PEO-b-PNIPAm underwent three stages of transformation during the heating process at 0.1 mg/mL: the disassociation of loose associates at temperatures below 28 degrees C, the micellization above 42 degrees C, and the aggregation in between. At each stage; the size and M-w,M-app exhibited distinct features. On the basis of these observations, possible mechanisms of the association and aggregation were also proposed.