Quality of power supply is now a major worldwide important issue making harmonic analysis an essential element in power system planning and design. Because of the power transformers are major components in power systems; this paper studies the impact of power harmonics on power transformer. This phenomenon increases the power losses, over loading, over voltage conditions; accelerate aging of life and failure probability of transformer. The paper refers to the importance of mitigation this phenomenon, and presents a typical case study of high voltage power transformer which exists in Egyptian Electricity Transmission Company (EETC). The electrical power network is simulated using the ETAP 6.0.0, and the results demonstrate the importance of mitigation of this phenomenon