Glucose oxidase (GOD) was covalently immobilized on the electroactive polyaniline (PAN) film with surface-grafted acrylic acid (AAc) polymer through the amide linkage formation between amino groups of GOD and the carboxyl groups of the grafted AAc polymer chains in the presence of a water-soluble carbodiimide. The surface structure and composition of the graft-modified and enzyme-functionalized PAN films were characterized by angle-resolved X-ray photoelectron spectroscopy (XPS). The amounts of immobilized GOD and its activity were also investigated. It was shown that the amount of immobilized GOD increased linearly with the concentration of surface-grafted AAc polymer chains. The decrease in activity of the immobilized GOD was considered to be due to, among other factors, the reduced accessibility of glucose molecules to the active sites of the enzyme and the conformational change of the GOD molecules as a result of the covalent immobilization. However, the immobilized enzyme was less sensitive to thermal inactivation as compared to that of the free form. The optimum pH value of GOD was not affected by the immobilization reaction and the pH stability range was considerably widened. The immobilized GOD also exhibits a significantly improved stability during storage in buffer solution over that of the free enzyme.