A pH-sensitive block copolymer is synthesized by step polymerization and its pH-sensitive micellization-demicellization behavior is studied. This polymer has a hydrophilic MPEG (shell) and hydrophobic but pH-sensitive poly(beta-amino ester) (core), which can form a self-assembled micelle. As confirmed by fluorescence spectroscopy and dynamic light scattering (DLS), this polymer shows a sharp pH-sensitive micellization-demicellization behavior. It is confirmed that the pH sensitivity is affected by the molecular weight ratio between the MPEG and poly(beta-amino ester).