Abstract: It is necessary that on-line liquid monitor system is installed in nuclear power plant. And it is an important measurement to be taken in evaluation on nuclear safety in nuclear facilities. This system consists of high pressure ionization chamber detector filled with Xe gas, 8051 single chip control unit and low current measuring amplifier. If the system shows the data per 5 min, and the confidence level is set to 95%, in the condition of natural radioactivity background, the detected limit is 2 644 Bq/m³.