Ascorbate oxidase has been immobilized in the polyaniline film by an electrochemical method. The response current of the polyaniline ascorbate oxidase electrode is slightly affected by the applied potential from 0.36 to 0.45 V vs. SCE; also the response current of the enzyme electrode is a little sensitive to pH from 6.26 to 7.02, although the immobilized ascorbate oxidase has an optimum pH at 6.8. The response current of the enzyme electrode at 0.40 V increases linearly with increasing concentration of ascorbic acid in the range 0.01-0.1 mM. Direct electrochemistry between the enzyme and the polyaniline film was observed. The narrow linear range between the response current and the substrate concentration is limited by the ascorbic acid diffusing through the polyaniline film at high concentration, then reacting on the underlying Pt. The response current of the enzyme electrode increases tremendously at both the phosphate buffer containing 0.125 mM ascorbic acid with pH 6.82, and phosphate buffer containing 0.1 mM ascorbic acid with pH 7.52. The enzyme electrode has a fast response time, and can be used to determine ascorbic acid at low concentrations.