In this research work, performed a partial discharge test on thermally aged liquids and solid insulating materials for analysing the withstand ability of insulating oil under applied electrical stresses. At first, breakdown strength, viscosity, flash point and fire point of oil samples have been measured for estimating critical characteristics of proposed oil’s to meet standard requirements as a liquid dielectrics for transformer. Consequently, an accelerated thermal aging test was carried out on oil samples at 100 C for 80 hours. However,for every 10 hours of aging period, the properties of oil samples have been measured for estimating the thermal degradation of oil samples. The statistical analysis has been made to analyse the failure probability of oil samples under high voltage stress. Further, rod – plane electrode and sphere – plane electrode configuration was used to simulate the partial discharge of solid insulating materials, although needle – plane electrode configuration is used to simulate the creepage discharge on oil samples. The experimental results show that thebreakdown strength and fire resistance characteristics of rice bran oil and sunflower oil are much higher than that of the mineral oil. After aging the critical properties of oil samples are gradually decreased. However, the rate ofreduction of critical properties in rice bran and sunflower oil is lower than that of others. Weibull distribution and statistical analysis found that rice bran oil’s deterioration rate is lower than that of others. Moreover, mineral oil has a lower creepage discharge voltage than other oils.