A voltammetric study of the interaction of 5-(4-diethylamino-2-hydroxyphenylazo)-4-hydroxynaphthalene-2,7disulfonic acid (beryllon III) with protein at the mercury electrode in Britton-Robinson (B-R) buffer solution is described in this paper. In pH 3.5 B-R buffer solution, beryllon III has a well-defined second order derivative linear sweep voltammetric reductive peak. After addition of human serum albumin (HSA) to beryllon III solution, the current value of reductive peak of beryllon III decreases while the peak potential does not shift and no new peaks appear. The electrochemical parameters such as the formal potential E8, the standard rate constant of the electrode reaction k s , the transfer coefficient a of beryllon III have no significant changes in the presence of HSA. So the binding interaction of beryllon III with protein forms an electrochemically non-active complex. The equilibrium constant for this super-molecular complex and the interaction mechanism are discussed carefully. On the basis of this interaction, a sensitive assay method for proteins, such as HSA, bovine serum albumin (BSA), egg albumin et al, is also established. The linear relationship between the decrease of the peak current and the concentration of the protein is in the range of 1.0 -70.0 mg/L with the detection limit of 1.0 mg/L HSA. This method was further applied to determine the healthy human serum samples and the results are consistent with the Coomassie brilliant blue G-250 spectrophotometric assay.