A small-diameter polymer composite tubing comprising polyethylene (PE) and poly(vinyl acetate) (PVAc) with a biocompatible surface was developed for application to medical devices. In this study, the following modification process was applied to the narrow PE tubing (inside diameter: 300 μm, outside diameter: 600 μm, length: 5.0 m). The polymer composite (PE/PVAc) was prepared by supercritical impregnation of a vinyl acetate monomer and initiator into PE tubing, followed by in situ radical polymerization within the polymer matrix. Infrared imaging measurement of a cross-section confirmed that PVAc was uniformly generated in the PE tubing. After the acetyl groups on the surface were hydrolyzed, a phospholipid polymer with both phosphorylcholine groups and silane coupling groups was immobilized onto the hydroxyl groups on the surface. These reactions were performed over the entire surface of the PE tubing with a high-aspect-ratio uniformly. This surface possessing the phospholipid polymer shows high hydrophilicity and a remarkable ability to suppress protein adsorption. This novel surface modification by molecular composite formation using supercritical CO2 can modify areas such as the lumenal surface of very narrow PE tubing which are difficult to modify in conventional modification methods. This new procedure could have applications for the preparation of new polymeric materials, including biomaterials.