Abstract: As a substitute for mineral insulating oil, silicone oil has been widely used in high voltage apparatuses, such as transformers and bushings. However, the research on the condition assessment of silicone oil paper composite insulation has been seldom carried out. In order to evaluate the effectiveness of frequency domain spectroscopy（FDS） method in assessing the insulating condition of silicone oil-paper composite insulating system, comparative study on the influence of thermal aging and moisture content on FDS property of both silicone oil-paper and mineral oil-paper system were carried out. The test results show that for both silicone and mineral oil-paper composite insulating systems, the moisture content has an obvious and similar affects on the FDS curves, the dielectric loss value of samples rises as the moisture content increases. The thermal aging affect the mineral oil-paper’s tanδ and ε_r greatly with a similar way as the moisture content, but it effect the silicone oil-paper tanδ and ε_r very little. Therefore, FDS method can be used to estimate the moisture content inside the silicone oil-paper composite insulation.