Abstract: Thermal expansion coefficients of insulating oils from various crude sources and different brands are studied in laboratory by ASTM D1903 method. Results show that the thermal expansion coefficients of most mineral insulating oils are in the range of 0.000 72 —0.000 80, having the trend of increase with the temperature slightly. For insulating oils from the same crude source and with great different viscosities, the thermal expansion coefficient of the high viscosity oil is smaller, and the low viscosity one is larger. For insulating oils with same viscosity level, the naphthenic base oil exhibits the least thermal expansion coefficient, followed by intermediate base oil and paraffinic base one is the most. For insulating oils from the same crude base, their thermal expansion coefficients decrease with the increase of aromatic contents.