Captopril, 1-[(2S)-3-mercapto-2-methyl-1-oxopropyl]-L-Proline, can be deposited onto a silver electrode by a covalent bonding method to give a long-lived and stable chemically modified electrode(CME). Since the CME is prepared with a reaction between captopril and the substrate silver, the CME being prepared by this method is very stable. Furthermore, Hemoglobin(Hb) exhibits excellent voltammetric response at the modified electrode. Differential pulse voltammetric(DPV) measurements of the protein with this CME reveal the existence of a linear relationship between the anodic peak current and the concentration of Hb in the range of 2×10−6∼ 5×10−5mol/L. The detection limit is 8×10−7mol/L and the relative standard deviation of results is 5% for 6 successive determinations at 2×10-5mol/L. The determination of Hb for a real example is carried out.