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The accelerated thermal experiments at 90degC, 110degC and 130degC are performed, using the samples that Kraft paper or thermally upgraded paper were immersed by mineral oil or nature ester. The results show that the leveling-off degree of polymerization (LODP) for ester-immersed paper insulation is higher than the one for mineral oil-immersed paper insulation, and the time of getting to the LODP for ester-immersed paper insulation is only one fourth to two thirds of the one for mineral oil-immersed paper insulation at the same aging temperature. The cause of resisting thermal aging performance for ester-immersed paper insulation was studied. There is a peak for the value of dissolved water in the ester for ester-immersed Kraft paper insulation, and the higher the temperature is, the higher the peak is and the shorter the time of reaching the peak is. The acid value abruptly increases when the value of dissolved water in the ester for ester-immersed Kraft paper insulation reaches the peak. The dissolved water in the ester for ester-immersed thermally upgraded paper insulation reduces at the aging process. The change of the micro-surface of the thermal-aged paper for ester-immersed paper insulation is slower than the one for oil-immersed paper insulation.