The world’s energy requirement has been dominated by petroleum oil for centuries in many application fields, including transportation, household, and electricity. Mineral oil as an important insulating material in transformers has been used more than 150 years. Mineral oil application in power system equipment can be potentially hazardous to the environment especially when there are any incidents during operational time like transformer explosion which may cause a spill of oil to the soil or water stream. Due to the concerns about the deducing petroleum resources and the environmental issues, the alternative insulating oil with biodegradable characteristics has been attracted lots of attentions for a couple of decades. In this thesis, the dielectric properties of a type of mineral oil and a type of vegetable oil were investigated, and the effect of accelerated thermal aging on their dielectric properties were studied as well. Since it affects its dielectric properties, such as breakdown voltage, dielectric dissipation, and relative permittivity, the humidity of oil was measured along the aging process. The experimental results were compared with those from the previous research, showing that most of the test results obtained in this work was comparable with the previous work.