We present here the synthesis of two kinds of amphiphilic block copolymers, a diblock copolymer MPEG-b-PTAm and a triblock copolymer MPEG-b-PLA-b-PTAm, which can self-assemble into micelles with nitroxyl radicals-containing PTAm segment in the core. The structure of the block copolymers was characterized by H-1 NMR and GPC. Dynamic laser light scattering and transmission electron microscopy were used to study the micellar behavior of the two block copolymers in aqueous solution. The micelles carrying nitroxyl radicals in the core can generate electron paramagnetic resonance, which is stable for a period of time up to 8 min even in the presence of reducing reagent such as ascorbic acid. The enhanced stability against the reducing agent was ascribed to the inaccessibility of the nitroxyl radical core placed in the interior of the micelles. Combined with the biocompatibility, these micelles were promising to be used as the EPR probes for bioimaging in vivo. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 5404-5410, 2010