Transformers are the largest part of the investments of transmission and distribution systems and have a considerable economic impact on the operation of an electrical network. It is advisable to know the insulation condition, operating life, and loading capacitance of a transformer to ensure its economical operation and secured service. The most important parameter in transformers life expectancy is the insulation temperature level which accelerates the rate of aging of the insulation. In this study, the top-oil temperature is studied both for indoor and outdoor transformers and the thermal models proposed for them are compared. Since the thermal transfer is different for indoor and outdoor transformers considering their operating conditions, their thermal models differ from each other. In this study, the top-oil temperature is analyzed for indoor and outdoor transformers and the proposed thermal models are verified by the results obtained from the experiments carried out on different operating conditions.