A facile method for the simultaneous measurement of tryptophan (Trp) and tyrosine (Tyr) was firstly exploited at unmodified boron-doped diamond (BDD) electrode. The experimental results indicated that by using differential pulse voltammetry, the oxidative peaks of these two kinds of amino acids could be completely separated at BDD electrode. The peak separation of Trp and Tyr was developed to be 0.64 V when Na 2 PO 4 /NaOH buffer solution with the optimized pH 11.2 was employed. The detection limit of Trp was obtained to be 1 Â 10 À5 M, while that of Tyr was achieved to be 1 Â 10 À6 M. The present method was also evidenced to be available to the determination of real samples of amino acids. Analysis of amino acids has been attracted more and more attentions because amino acids, as the rudimental units of enzymes and proteins, are so closely related to the life. Up to now, a number of analytical methods, such as high performance liquid chromatography [1 -3], electrophoresis [4 -6], gas chromatography [7 -8], ion exchange chromatography [9 -10], spectroscopy [11 -13] and etc. were developed. However, there somewhat are limits in these methods, e.g., low sensitivity and poor selectivity, high cost, easy to deactivation by fouling and etc. So, it is a challenging research work to establish an analytical system for the direct and swift determination of amino acids with low-cost [14 -15].Several research works on the electroanalytical determination of some amino acids, including Trp and Tyr have been described in the last few years. The mercury electrodes [16] and carbon paste electrodes [17] have mainly been employed in these researches. More recently, carbon paste electrodes with electrochemical pretreatment [18] or special chemical modification [19 -20] have been developed in electroanalysis of amino acids with good performance. Unfortunately, it is still some difficult to realize the simultaneous determination of above two kinds of amino acids by direct voltammetry at those mentioned electrodes because oxidative peaks of amino acids could not be completely separated.BDD is one of the most popular electrode materials [21]. It exhibits several excellent electrochemical properties, including a wide electrochemical potential window in either aqueous or non-aqueous media, very low capacitance, and high electrochemical stability which ensures BDD with a good reproducibility. Superior to glassy carbon electrodes, it is easy to refresh the BDD electrode surfaces and it shows extremely excellent reproducibility. In addition, BDD electrodes have recently been found to show high resistance to fouling and insensitivity to dissolved oxygen. These properties make BDD a promising material for electroanalytical applications. BDD electrodes with electrochemical pretreatment or modification have been employed for the simultaneous measurement of uric acid (AA), dopamine (DA) and other biomolecules with good selectivity and sensitivity [22 -25].In the present work, we firstly realized the simultaneous and direct voltammetric...