Testing of the 400MVA transformer of the Bandar Abbas Thermal Power Plant was carried out as one of the most important maintenance and operation measures. These tests are carried out to check the electrical, mechanical performance, and insulation condition of the transformer. Running tests plays a key role in ensuring network stability and preventing widespread outages. At the Bandar Abbas Thermal Power Plant, testing of the 400 megavolt-ampere transformer is carried out in accordance with international standards. The results allow for early identification of potential defects. Ultimately, performing accurate tests will increase the life of the transformer and reduce repair costs.