An antibiotic drug, d-cycloserine (CYS), in phosphate buffer at different pH values was electrochemically studied using a gold electrode. A well-resolved, irreversible, diffusion-controlled voltammetric peak was obtained at 0.873V in a pH 9.2 phosphate buffer solution with an ionic strength of 0.2M. The experimental conditions, such as the solution pH and scan rate, were optimized. The pH dependence suggested that equal numbers of protons and electrons were involved in the electrochemical oxidation of CYS. A plausible oxidation mechanism was proposed. Under the selected conditions, the dependence on the concentration of CYS revealed that the detection limit for CYS was 3.3×10−8M for differential pulse voltammetry and 4.91×10−8M for square wave voltammetry. The proposed method was successfully applied to the determination of CYS in drug capsules and urine samples. The electrode exhibits good reproducibility and stability. In addition, the influence of various foreign species was studied.