A nanocomposite consisting of multi-wall nanotubes (MWNTs) grafted with a biocompatible polymer poly(2-hydroxyethyl methacrylate) was prepared by in situ polymerization in supercritical carbon dioxide. The surface of the MWNTs was first surface modified with hydroxyl groups in the solution of KMnO4 and a phase-transfer catalyst. MWNT-OH was then functionalized with vinyl groups using a silane coupling agent, -methacryloxypropyltrimethoxysilane. The silane groups can improve the dipersion of MWNTs in supercritical carbon dioxide, while the terminal vinyl groups help fabricate polymer chains on the MWNT surface. The as-synthesized products were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermo-gravimetric analysis. The SEM and TEM images showed that the nanotubes were well coated with the polymer shell. The composite had higher thermal stability than the pure polymer and dispersed well in methanol. This biocompatible polymer composite was prepared using a green method and is expected to be useful as a biomaterial composite with potential applications in the biological field.