Increasing of load demand causes the greater electrical power needed. When the transformer operation in overload conditions and hottest-spot temperature, it will accelerate the aging and failure in oil insulation of transformer. Application of distributed generation (DG) can provide the additional electrical power supply to the loads (household). So that, it can relieve the workload power transformer and reduce the acceleration of aging in oil insulation of transformer. This research will analysed the effect of loading capacity with DG injection using a PV generator on the condition of oil insulation on 150 / 20kV of electricity distribution system using ETAP software. Tests of this research are carried out on 24 hours of loading conditions on the distribution system. The calculation of isolation oil temperature was carried out using a mathematical formula according to the IEEE, and oil insulation would be tested with a breakdown voltage (BDV) test.