A simple, sensitive aptamer-based biosensor for the detection of phenylalanine is developed using the electrochemical transduction method. For this proposed aptasensor, a 5-thiol-terminated aptamer is covalently attached onto a gold electrode. At the first time, the electrode was evaluated as an electrochemical aptasensor for determination of phenylalanine in aqueous solutions. This sensor was tested in a Tris-HCl buffer with physiological pH=7.4 by cyclic voltammetry and differential pulse voltammetry. The detection limit and sensitivity of the modified electrode toward phenylalanine were estimated to be 1 nM (S/N=3) and 0.367 mu A nM(-1), respectively. The linear range of the signal was observed between 1 and 10 nM of phenylalanine with 0.9914 correlation factor. The herein-described approach is expected to promote the exploitation of aptamer-based biosensors for protein assays in biochemical and biomedical studies.