Amphiphilic copolymers consisting of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) and hydrophobic monomers are known as biomaterials for the administration of poorly water-soluble drugs such as paclitaxel (PTX). However, the hydrophobic monomers to be copolymerized with MPC have not been optimized for PTX solubilization and its dosage forms. Here, we show the enhanced PTX solubility by only an MPC-based amphiphilic copolymer using a polyhedral oligomeric silsesquioxane (POSS) methacrylate (MA) bearing an ethyl (C2H5) group as a vertex group. MPC was copolymerized with POSS methacrylates bearing different vertex groups of ethyl (C2H5), hexyl (C6H13), and octyl (C8H17) via radical polymerization. We found that the strong interaction between C2H5-POSS and PTX contributed to the slow release of PTX without any burst release. The C2H5-POSS-MA MPC copolymer was internalized into the cultured HeLa cells, which was confirmed by using a fluorescein-4-isothiocyanate (FITC)-labeled PTX, and the PTX-dissolved copolymer induced cell death. We anticipate that the C2H5-POSS-MA MPC copolymer is a good solubilizer bearing a controlled release function for PTX.