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Cysteic acid based on electrochemical oxidation of L-cysteine (CySH) form a novel thin film material at a glassy carbon electrode (GCE) for electroanalytical determination of sinomenine. The determination of sinomenine at the modified electrode with strongly accumulation was studied by differential pulse voltammetry (DPV). The peak current obtained at + 0.650 V (vs. SCE) from DPV was linearly dependent on the sinomenine concentration in the range of 1.0 × 10-6 ~ 1.0 × 10-4 M in a B-R buffer solution (0.04 M, pH 1.81) with a correlation coefficient of 0.999. The detection limit (S/N = 3) was found to be 6.0 × 10-7 M. The low-cost modified electrode showed good sensitivity, selectivity, stability and had been applied to the determination of sinomenine in pharmaceutical formulations with satisfactory results. The electrochemical reaction mechanism of sinomenine was also discussed firstly.