Concentration dependent morphology of 3-armed poly(ethylene glycol)-b-poly(epsilon-caprolactone) copolymer aggregates in aqueous system was investigated by atomic force microscopy (AFM). The AFM results show that, at a low concentration, 4 x 10(-5) g/mL, spherical micelles occur, and unmicellized molecules are not distributed homogeneously in the copolymer aqueous solution. Unequal outspread clusters composed of wormlike aggregates are formed at a moderate copolymer concentration, 4 x 10(-4) g/mL, those wormlike aggregates are orderly packed in the clusters. At a high concentration of 0.05 g/mL, the copolymer aqueous system is indeed a gel at room temperature, outspread clusters of wormlike aggregates join together to forma network structure. (C) 2008 Wiley Periodicals, Inc.