Power transformers represent the largest portion of capital investment in transmission and distribution substations. One of the most important parameters governing a transformer’s life expectancy is the hot spot temperature value. The aim of this paper is to introduce hot-spot and top-oil temperature model as top oil and hot spot temperature rise over ambient temperature model and thermal model under liner and non-linear loads. For more accurate temperature calculations, in this paper thermal dynamic model by MATLAB is used to calculate the power transformer temperature. The hot spot, top oil and loss life of power transformer under harmonics load are calculated. The measured temperatures of 25 MVA, 66/11 kV, ONAF cooling temperatures are compared with the suggested dynamic model.