The self-assembling property of an amphiphilic rod rod diblock copolymer has been demonstrated using poly(n-hexyl isocyanate)-block-poly(2,5,8,11-tetraoxatridecyl isocyanate) (PHIC-b-PEOIC) with different degrees of polymerization for the PHIC and PEOIC segments. The critical aggregation concentrations (CAC) of PHIC-b-PEOIC decreased with the increasing fraction of the hydrophobic PHIC segment. The relation between the hydrodynamic radius (R-h) and the PHIC/PEOIC ratio differed from that for the radius of gyration (R-g). The self-assembled PHIC-b-PEOIC was stable in various polymer concentrations. The rho parameter (R-g/R-h) strongly suggested that the macromolecular architecture was a cylindrical micelle for PHIC13-b-PEOIC41 (rho = 1.63) and a spherical vesicle for PHIC22-b-PEOIC36 (rho = 1.09) and PHIC31-b-PEOIC31 (rho = 1.09). Transmission electron microscope (TEM) images closely agreed with the structures of the cylindrical micelle and spherical vesicle, which were expected based on the rho parameter.