Poly(ethylene imine) (PEI)-immobilized surfaces have attracted much attention as interesting biomaterials in relation to cell culture, selective adsorption of endotoxins, and so on. In this study, we found that PEI could be effectively immobilized on various polymers pretreated by oxygen plasma. Surface analyses using an X-ray photoelectron spectrometer and contents of peroxide groups determined by iodide method indicate that the hydroxy peroxides were:the main functional groups that contributed to PEI immobilization covalently on the polymer surfaces treated with oxygen plasma.