Amphiphilic block copolymers based on azido-functional polystyrene (PVBA) and hydrophilic poly(ethylene oxide) (PEO) have been synthesized by nitroxide mediated polymerization and used to prepare aqueous spherical micelles. These micelles consisting of a PEO corona and a PVBA core were stabilized by cores cross-linking under UV irradiation without addition of photoreagent. The efficiency of the core cross-linking was estimated in term of the conversion of azide groups to dibenzyldiazene junctions with liberation of gaseous nitrogen molecules. The micellar structures were studied by dynamic light scattering (DLS) and atomic force microscopy (AFM). Differential scanning calorimetry (DSC) was used to characterize the thermal behavior of freeze-dried micelles. Attachment of PEO block to a rigid core modified considerably the thermal behavior of PEO. We observed a decrease of about 15 °C for the crystallization temperature and 50% for the degree of crystallinity of cross-linked micelles submitted to 45 min UV irradiation compared to pristine copolymer.