A significant part of power transformers, currently in Latvia has fulfilled the normative life. Economic situation and total number of equipment with long maintenance service do not allow replace it in the coming years. Therefore, to maintain the required and necessary reliability of transformers is very important diagnostic checking and if necessary should to make big repairs or modernization works. The article describes the important aspects of the preparation of technical decision making. Before modernization works of power transformers necessary to realize complex diagnostic. Complex diagnostic consists of seven main stages. Comprehensive diagnostic tests can identify not only developing defects, determine their level of risk, but also justify the need, scope and timing of modernization works.