A miniaturization micro volume auto titrator coupled with electrochemical end-point detector was designed and fabricated for the determination of acidity of some Thai citrus fruit juices collected in the northern area of Thailand. The method was based on on-line potentiometric titration of the acid contents with sodium hydroxide. Conditions such as volume of fruit juice sample, volume and concentration of potassium chloride used as supporting electrolyte and flow rate of titrant were optimized by using univariate optimization. A sample throughput of 83 samples h-1 at 0.28 mL/min was achieved with satisfactory results. The results obtained by the proposed method agreed with those obtained by using the standard classical titration method.