Abstract: A poly ( tryptophan) modified glassy carbon electrode ( PTRY/GCE) was prepared by cyclic voltammetry. The electrochemical behavior of diethylstilbestrol ( DES) at the fabricated electrode was studied.A new method for the determination of DES in animal food was established.The experimental results showed that in p H5.0 phosphate buffer solution ( PBS) system, DES appeared a pair of quasi redox peaks in the potential range of-0.2 to 0.8 V on the PTRY/GCE. The modified electrode showed a catalytic effect to the electrochemical reaction of DES.The oxidation peak current was proportional the concentratio of DES with a linear range of 5.00 × 10^-8 to 1.00 × 10^-6 mol/L and a limit of detection was 2.00 × 10^-9mol/L. The RSD for 6paralled determinations was 2.4%.The poly ( tryptophan) modified glassy carbon electrode had good sensitivity, selectivity, and stability.The method could be applied to the determination of DES in beef, fish and milk powder with recoveries of 96.0% ¹02.3%.The results were satisfactory, and the method had practical value.