The micellar characteristics of the amphiphilic diblock copolymers based on a hydrophobic polyphosphazene (PN) and hydrophilic poly(ethylene oxide) (PEO) were investigated by using fluorescence techniques, dynamic light scattering, and transmission electron microscopy. The two diblock copolymers with different ratios of the length of block components form spherical micellar aggregates in an aqueous phase. The critical micelle concentrations of PN-PEO 1 and PN-PEO 2, determined by a fluorescence technique, were 12.4 and 5.2 mg/L, respectively. The mean diameters of the micelles of PN-PEO 1 and PN-PEO 2, measured by dynamic light scattering, were 100 and 120 nm. The steady-state fluorescence anisotropy values (r) of 1,6-diphenyl-1,3,5-hexatriene (DPH) were 0.103 for PN-PEO 1 and 0.108 for PN-PEO 2. The low anisotropy values for PEO-PN compared to those for other polymeric amphiphiles may be due to the intrinsic flexibility of the trifluoroetboxy-substituted polyphosphazene block, which would allow enhanced rotational diffusion of the probe.