Thiamazole is covalently attached onto the surface of a silver electrode to form a chemically modified one, which is long-lived and stable. It shows the capability of selectively complexing myoglobin and can be applied as a selective biosensor for this compound. Myoglobin is accumulated onto the CME at 0.32V (vs.SCE). After exchanging the medium, it is determined by differential pulse stripping voltammetry. A cathodic stripping peak is obtained by scanning the potential from 0.32V to 0.0V. The peak currents are linearly proportional to the concentrations of the protein in the range of 0.5∼5.0mg/ml. The relative standard deviation is 6.4% for 6 successive determinations of 3.0mg/ml myoglobin.