A MATLAB SIMULINK IEEE and thermal models has been established to determine the transformer hot spot and oil temperatures. The models are APPLIED on 25 MVA, 66/11 kV ONAF cooling transformer units at varying load and the results are compared to the measured temperatures results. It is shown that the thermal model yield results are in agreement with the measured results, especially for the top oil temperature. The results obtained by the IEEE model are also very good for the hot spot temperature calculation but less accurate for the top oil temperature. The calculated top oil and hot spot temperatures with and without harmonic loads are calculated under constant load the shows that top oil temperature in transformer with harmonic current is greater than without by ten degrees and hot spot temperature by thirteen degree. The increase in the transformer temperature would lose all of its life in half of its chosen normal life.