Abstract: In this paper, the oil-paper insulation system of the power transformer is analyzed in the frequency domain. Different factors that influence the dielectric insulation of oil-paper, such as the temperature, moisture in cellulose, conductivity, and oil aging, are described. The analysis for the dielectric response is carried out by analyzing the dissipation factors and the complex capacitance through the Frequency Domain Spectroscopy (FDS) method. This tool analyzes the dielectric responses of the oil-paper insulation system, by measuring a series of dissipation factors at different frequencies, typically distributed over the range of 0.001 Hz to 1000 Hz. The result of the experiments shows that the temperatures, moisture in cellulose, conductivity, and oil aging have a significant influence, in the low-frequency range than in the high-frequency range.