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Frequency domain spectroscopy (FDS) has been used to assess the ageing condition of oil-paper insulation used in transformers. To further understand the ageing process, the reduction in degree of polymerization (DP) of cellulosic paper in itself on the dielectric response was investigated first. It has been found that the reduction in DP itself can alter the dielectric characteristics. The oil-paper insulation was thermally accelerated aged at 110 °C for up to 154 days according to the procedure described in IEEE Guide. Then dielectric characteristics of the thermally aged samples were obtained at different measurement temperatures. The results reveal that the relative permittivity (εr') plots of oil impregnated pressboards increase with the reduction in cellulose polymer chain length. εr' and tanδ values of oil impregnated pressboards shifts upwards in the lower frequency range (10-3 Hz -10-1 Hz) with different ageing condition, indicating the possibility of utilizing the characteristics of εr' and tanδ values at lower frequencies (10-3 Hz-10-1 Hz) to quantitatively characterize ageing condition of oil impregnated pressboard. The parameters including DP, εr' and tanδ values at the three characteristic frequencies (10-3 Hz, 10-2 Hz, 10-1 Hz) have been analyzed as a function of the ageing time, showing an exponential relationship based on the best fit. In addition, the εr' and tanδ curves of aged oil impregnated pressboard have been observed to be shifted upwards to higher values at lower frequencies when the measurement temperature is increased. A shifting factor αT which describes the relationship between frequency and temperature has been defined. It allows one to shift the master curve of impregnated pressboard with DP obtained at- reference temperature to other testing temperatures.