This study investigates the effect of 1-Methylenaphthalene and Benzophenon as additives in cable oil used in extra high voltage paper insulated underground cable in service in Riyadh area. Samples containing different concentrations of these additives were used. Impact of variation of temperature and additive concentrations was evaluated for conduction current and burst current. Virgin samples of the dielectric paper used in this cable were impregnated in these oil samples for the investigation of power factor and breakdown voltage. The results indicate that an optimum concentration of the additives improves the performance of paper-liquid composite.