In this Thesis project I dealt with the thermal ageing of polyethylene insulated cables. In the first semester I started my work with literary research, I reviewed the elemental strains on electrical insulations, its causes and effects, the electrical examine methodes used for examination of insulations. It was necessary, because the occuring processes in the cable can be understood only by reviewing of the basical physical mechanisms in appropriate detail. After I studied the mechanisms of the ageing of polyethylene. Abroad several experts have dealt with the ageing of polyethylene insulation. I summarized the important results in one chapter. I applied The department’s Return Voltage Measurement (RVM) device for determine cables’s whole polarisation spectrum. The measurements were started to run on unaged samples, than I continued the process for eight aging cycles. At the evaluation I worked with the measured return voltage (Ur) and the slope of the initial value of the return voltage (Sr).