One of the basic pillars of the safe and reliable operation of the nuclear power plants is the proper condition of the safety related cables. One of the most important issue to maintain safety is the condition monitoring of the instrumentation and control cables to avoid the failures and accidents. In my thesis, I evaluated the results of the insulation resistance measurement, which was carried out as a part of the International Atomic Energy Agency Coordinated Research Project (IAEA CRP). According to the results of this project the insulation resistance measurement cannot be used as a condition monitoring method. In contrast, in my work I elaborated evaluation criteria which are different from the methods of the standards and enable the insulation resistance measurement can be a condition monitoring technique. The measurements were executed on thermal and radiation aged EPR, PEEK and XLPO insulated cables. The results suggest that the slopes and the cut off points of the leakage current-time curves show strong correlation with ageing in the case of EPR and XLPO insulated cables. According to these results, the insulation resistance measurement can be applied for condition monitoring method for nuclear power plant cables.