Measurement of complex dielectric properties of materials has attracted a lot of attention in various applications. The need for transformer oil as insulating and cooling equipment has increased, in high-voltage power transmission network. In this paper, the frequency response of transformer oil has studied through dielectric constant measurements using waveguide cavity resonator and transmission/reflection method. Measuring with transmission/reflection method has done for transformer oil before and after process. The results show that the frequency response of oil before and after use in transformers will be different. So this method can be proposed for aging and quality control.