A triblock poly(2-methacryloyloxyethyl acrylate)-b-poly(ethylene oxide)-b-poly(2-methacryloyloxyethyl acrylate) (PMEA-b-PEO-b-PMEA) copolymer bearing polymerizable groups on the hydrophobic blocks synthesized and self-assembled in water. The transient micellar structures formed in selective solvent were permanently fixed by cross-linking the methacrylate moieties in micelles cores under UV light. The structures obtained after cross-linking were characterized by static and dynamic light scattering (SLS and DLS), size exclusion chromatography (SEC), and electron microscopy (TEM and cryoTEM). SLS measurements showed an increase of the aggregation number of the micelles during the photo-cross-linking process. This result was discussed in terms of competition between the micelles polymerization rate and the unimers exchange rate. At low concentration, mainly flower-like architectures were obtained. As the concentration of the cross-linked solutions increased, more complex aggrega! tes, in which the "flower" is the basic unit, were obtained.