Diagnostics is very important moment in determination of transformer technical condition. Accurately and careful determination is possible only if there is special technical staff with appropriate equipment. The insulation is very important component in electrical equipment. The insulation determines both the lifetimes of equipment and the economical aspects of power supply. Therefore it is important assess the condition of insulation. The insulation system of equipment consists of several materials that all have different dielectric properties. Necessary to make a choice and get very good diagnostic program witch can show all oil-paper properties. The oil has an effect on transformer isolation at all. Cause in that moment, when transformer is measured is necessary take an oil sample too. Only after checking of oil sample, can say about oil condition and to answer technical requirements.