Power transformer has an important role in a transmission system. One factor that can reduce efficiency in the transmission system is transmission losses. Efficiency can be increased by reducing losses in the power transformer. The biggest influence determining the losses on the transformer is the material characteristic used by the transformer itself. To get maximum efficiency, it is necessary to optimize the materials on the transformer to get the lowest losses. The study is conducted by measuring core losses and copper losses on the 60 MVA 150/20 kV power transformer with the specified specifications. Furthermore, the transformer materials are changed into several types and the losses calculations are done. The calculation results are compared to get the most effective material. The results show that in the calculation of core losses, the core material type with the lowest core loss value is NK E-core of 0,49 Watt/kg. While in the calculation of copper losses, the winding material type with the lowest loss value is Continuously Transposed Conductor.