Abstract: Glassy carbon electrode (GCE) modified with composite membrane of poly-o-phenylene diamine and L-tyrosine (poly-o-PD-Tyr/GCE) was prepared by electropolymerization of o-phenylene diamine onto the L-tyrosine functionalized GCE which was obtained by covalent bonding of L-tyrosine to the surface of GCE through C-N bond via electro-oxidation.It was found that in a PBS of pH 6.8,the modified electrode,poly-o-PD-Tyr/GCE,behaved to catalyze the electro-oxidation of ascorbic acid at the oxidation potential of 0.35 V,which is 0.23 V lower than that at the bare GCE.Linear relationship between values of oxidation peak current (Ip) and concentration of ascorbic acid was obtained in the range of 2.5×10-4-1.5×10-3mol·L-1.Detection limit (3s/k) of the electrode toward ascorbic acid was found to be 43.64 μmol·L-1.No significant change of current response was observed within 5d when the electrode was kept in 0.1 mol·L-1 PBS (pH 6.8).Value of RSD (n=10) of 3.2% was found when precision of measurements with the electrode was tested with ascorbic acid solution of 5×10-4mol·L-1.