Dielectric subsystems have an important influence on reliability, lifetime and power of electrical equipments. Study of dielectric parameters and their changes due to degradation mechanisms provide important information about the equipments. Paper deals with oil-paper systems and their thermal degradation. These systems are used for example in transformer, capacitors and bushings. Thermal degradation causes increasing of moisture content thus number of polar particles. Therefore the main idea of this paper is diagnostic of oil-paper insulation based on a method of polarization measurement. One of the methods is Cole-Cole diagram [1] or its modification e.g. Davidson [5], Havriliak-Negami [11]. The next aim of the paper is measurement of the system parameters (Complex permittivity, Viscosity, Acid value, Resistivity, Polarization index and Dissipation factor) and their changes due to thermal degradation. In experiments are used a normal and an environmental friendly fluid.