A transformer is an electromagnetic device which works on principle of Faraday´s Law of induction. Transformers are used to transform the voltage level through a point in the power grid. Transformer or electrical insulating oil is an oil with special characteristics that is stable at high temperatures and has good insulating properties. The role of oil in transformers is cooling, insulation and stopping sparking and corona discharge. Also transformer oil protects cellulose insulation by preventing direct contact between cellulose and atmospheric oxygen. It also protects coils that are completely immersed in oil. There are two main types of transformer oils; mineral oils that can be inhibited and uninhibited, and synthetic oils. In electrical insulation oils, we observe electrical, physical and chemical properties. In this paper, the density and ultimate surface tension of the oil will be particularily monitored. It is important to measure the density of transformer oils especially because the oil should never be havier than water, because at very low temperatures ice could float to the surface of the oil. By measuring the surface tension of the oil, we can detect the presence of polar pollutants and oil oxidation products. In this final work, samples of transformer oils from transformers in operation and one new oil were examined. The characteristics of eight different oil samples from different manufacturers were measured.