Amphiphilic diblock polymers having an octadecyl group as a hydrophobic segment with different degrees of polymerization were synthesized by living cationic polymerization. Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI TOF MS) was performed to determine the molecular weight. Differential scanning calorimetry (DSC) measurements for polymer solid samples showed the melting point depending on the degree of polymerization. The size and shape of micelles formed by the polymers in water were investigated by small-angle X-ray scattering (SAXS) and neutron scattering (SANS) measurements. The aggregation number reduced with increasing degree of polymerization, while the overall micelle size was almost independent. The SAXS and SANS data revealed the sphere-to-disk transition on changing temperature for the polymer with the shortest hydrophilic chain. Below the melting point of hydrophobic chain, the polymer formed disklike aggregates with a crystallized core of octadecyl groups surrounded by a swollen shell. With increasing temperature, octadecyl groups melted and the spherical micelle formed.