Manufacturing process of oil immersed Power Transformer (PT) has significant effect to the value of its partial discharge (PD). PD is one of the most important parameters of PT to ensures the reliability and quality of the PT's operation PT. As PT is one of the most important parts in electrical power system, PD becomes one of the key performance indicators for PT. The quality of manufacturing process of both liquid and solid insulations will reflect to the PD value of PT. Both solid and liquid insulation will form the overall insulation of integration of PT. One of the important manufacturing processes is vacuum and oil impregnation process. These processes ensure the removal of the moisture from PT insulation and keep maintaining PT from moisture ingress. In this study, the experiment is conducted to investigate the relation between vacuum and impregnation process duration to partial discharge value of 167 MVA 500/150 kV on single phase power transformer. Twelve (12) samples are investigated during manufacturing process and final acceptance testing by measuring vacuum and impregnation process duration as well as the value of partial discharge. The result shows that vacuum process duration has significant effect to PD values. Combined with oil impregnation process of the insulation, the results show that to achieve PD below 70pC (<70pC), the optimum vacuum duration is 57 hours and the oil impregnation duration is ranged 75-120 hours. This practical study is beneficial for both the owner and the manufacturer of PT, so it gives the optimum duration for manufacturing process to get lower partial discharge at optimum duration and cost.