This paper reports the potentiometric measurement of ascorbic acid in the solution of 0.10 mol/L NaOH-0.1 mol/L NaCl using carbon paste (the mixture of spectroscopic graphite powder and di-iso-octyl phthalate) electrode(room temperature 15°C), with the linear range 7.0 × 10−7× 4.0 × 10−5mol/L, average response slope 95mV/decade and detection limit 1 × 10−7mol/L. Phenol, sulfite, Mn2+and so on pose no interference to the measurement of ascorbic acid. This method is characterized by fine selectivity, reproducibility and accuracy. The potential response behavior is caused chiefly by chemisorption of ascorbic acid to the surface of the carbon.