Cross-linked polyethylene (XLPE) is increasingly used for medium to high voltage distribution cables for their excellent mechanical, thermal and electrical performance. However, the existence of impurities/voids inside this insulting material represents the major problem when it is used as insulation in cables due to the occurrence of partial discharge (PD). Therefore, this study investigated the effect of existence of such impurities/voids inside 18/30 kV XLPE cables on the activities of PD. Moreover, the effect of changing the cavity position, size and geometry on the partial discharge behaviour was studied. Also, the potential gradient and the electric field distribution across this insulating material were obtained and analyzed during different operating conditions using COMSOL program. The simulation results of our study are nearly matched with measurements performed by others related to our study. The simulation results verified that; nearest the cavity to the core causes higher electric field potential and decrease with increasing the cavity distance from the cable core. This is in addition to the larger the size of cavity the higher the electric field potential and consequently fasting the activities of PD.