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Moisture content in transformer oil is an important index for equipment condition. Based on technical requirement of on-line monitoring of moisture content, this paper presents a water solubility estimation method using least square technique. Water distribution and solubility in oil is analyzed, and the Karl-Fischer method is adopted to determine moisture content in one oil sample; then, temperature and relative saturation are collected by temperature and moisture sensors. According to the empirical relation of water solubility and temperature, a formula of water solubility is finally determined using least square method, which ensures the measurement accuracy of moisture content in on-line monitoring application.