During the operation of electric power transformers,their insulation often made from paper and mineral oil is subjected to thermal, mechanical, and electrical stresses. Therefore, the lifetime of power transformers can be assimilated with their insulation system lifetime. The aim of this paper is to explore and present main degradation aspects for mineral oil, subjected to the action of high electrical field and partial discharge and exposure to thermal stress exclusively, analyzing and pointing out changes in the dielectric properties. Thus, the obtained results indicate that both the thermal and, especially, the electrical stresses produce important variations of the values of the real and imaginary part of the complex relative permittivity, the electrical conductivity, and the loss factor which indicates a severe oil degradation.