Transformers are considered as one of the main and high cost components of the power systems. This is due to the fact that their failure may have negative influence on sustainability and quality of energy. In addition, some failures may lead to high cost for replacement or repair and an unplanned outage of a power transformer is highly uneconomical. As a result, as a major equipment in power systems, its correct functioning is vital to enable efficient and reliable operation of electric power system. A transformer can fail due to any combination of electrical, mechanical or thermal stresses. The normal operation life of a transformer is partially related to the deterioration of its insulation through thermal ageing, which is determined mainly by its daily cyclic loadings. In this paper, heat analysis of a 1.5 MVA Oil-Natural Air-Natural cooling mode distribution transformer is experimentally investigated by equipped fiber optic sensors. The density of the fiber optic sensors is increased at the active parts in critical spots. Temperature measurements are realized according to the transformer losses variation and load variation tests. The thermal behavior of the distribution transformer is obtained. Contrary to the existing studies, the obtained results show that also the structure and the design of the transformer affects the hot-spot position and temperature. (C) 2017 Elsevier B.V. All rights reserved.