In this paper two types of oil-paper samples are thermally aged in two temperatures of 120. The samples are constituted by 25# oil, 45# oil and paper. Some aging indicators are measured during the aging process. Three parameters are measured during the aging process. The insulating ability of paper can be evaluated by water content and CO2/CO value. The model of functional relation for degree of polymerization (DP) with CO2/CO value, water content and service period are established by multiple linear regression analysis.