Abstract: The oil-paper insulation system in power transformers suffers from both heat and electricity. The current research mainly focuses on the characteristic of electrical failure,thermal failure,electrical or thermal aging on the oil impregnated paper,but the investigation on the electricalthermal aging is insufficient. In this paper,the polymerization degree, tensile strength and frequency domain dielectric spectroscopy properties of oil-paper structures at different aging levels are investigated through studies on the electrothermal combined aging of oil-paper insulation. The fitting formula of second order model of DP under electrothermal combined aging condition and the relationship between DP and tensile strength are obtained. A further study of dielectric spectrum in frequency domain shows that εr and tan δ in low frequency region increase with decreasing frequency with increasing aging time. Between10-3-10-1 Hz frequency range,εr and tan δ are obviously correlated with the aging state of the insulating paper,and the relationship between the values of εr and tan δ at 10-3,10-2and 10-1Hz characteristic frequencies and the degree of polymerization of insulating paper is obtained. This study provides a scheme for the electrical-thermal aging mechanism of oil-paper insulation and the application of FDS to nondestructive evaluation of transformer oil-paper insulation status.