Abstract: Polymethylene blue modified pencil core electrode (PMB/PCE) was prepared by electropolymerization of methylene blue (MB) onto the surface of PCE through scanning cyclic-voltammetrically for 50 cycles at a rate of 100 mV·s-1 in a solution containing 0.01 mol·L-1 MB in 0.1 mol·L-1 PBS of pH 5.0 and 0.2 mol·L-1 KCl solution.A pair of redox peaks was observed in the cyclic voltammogram under above mentioned condition,and the values of peak potantial (Ep) were shifted negatively as the pH-values of the solution were increased,and linear relationship was kept in the pH range of pH 1.90 to pH 8.52,or more precisely in 2 segments,one from pH 1.90 to pH 6.76 and the other from pH 6.76 to pH 8.52.Acidity of sample juices of orange,pineapple and cucumber were determined cyclic voltammetrically by using PMB/PCE as indicator electrode,platinum wire as opposite electrode and Ag/AgCl electrode as reference electrode.It was shown that more accurate and precise results were obtained by the present method as compared with those found by conventional pH-meter using glass indicator electrode or by the precise pH-paper.