An amphiphilic polyphosphazene diblock copolymer (MEEP-Ph/MEEP) based on [N=P(OCH2CH2OCH2CH2OCH3)(2)](n)(MEEP) as a hydrophilic block and [N=PPh(OCH(2)CH2OCH(2)CH(2)OCH(3))](m)(Ph/MEEP) as a hydrophobic block was synthesized via the controlled cationic block copolymerization of phosphoranimine. The molecular weight of the block copolymer was estimated by GPC to be 3300. The molar composition ratio of repeating unit in the MEEP block to those in the Ph/MEEP block was 0.5. The micellar characterization of MEEP-Ph/MEEP in an aqueous phase was carried out by using fluorescence techniques, dynamic light scattering, and TEM. The block copolymer formed micelles with a critical micelle concentration (cmc) of 80 mg/L. The hydrodynamic radius (R-h) of micelles, measured by dynamic light scattering, was 120 nm at 25 degreesC with a narrow distribution. The TEM image showed micelles with a spherical shape. Intermicellar aggregation was induced in the micellar solution of the block copolymer above 57 degreesC. The partition equilibrium constant, K-v, of pyrene in the micellar solution of the block copolymer was 7.0 x 10(3). The steady-state fluorescence anisotropy value (r) of 1,6-diphenyl-1,3,5-hexatriene (DPH) was 0.237. The fluorescence lifetime value for DPH in the block copolymer solution showed that the core region of the micelles consisted of two regions with different hydrophobicities.