Transformer has an important role in electrical power system, because it is used as voltage adjuster for the load served. Therefore the transformer must be protected and maintained to have live longer. In this final project, research transformer’s age estimation that is affected by various variations of load, temperature of transformer and ambient temperature of transformer in bantul substation 150 KV based on the 2009 IEEE standard. In this research use descriptive analysis method, which is a research method carried out by collecting data directly then processed into calculations and then analyze and take conclusions that can be used as a basic process to determine the shrinkage life of the power transformer due to loading. The calculation of the transformer starts from calculating the loading ratio then used to calculate the increase of the top oil stable temperature and the increase of the top oil temperature for changing load and then calculate the temperature difference between the hotspot and top oil. Then calculate the hotspot temperature use Yogyakarta environmental temperature data. From the calculation of the temperature of the hotspot can be calculated the relative thermal aging rate and then the results to calculate shrink age and remainder age of the transformer. The result of the research acquired that the highest estimated age is 33,55 years at load 61,52% with shrinkage of transformer age 0,523 p.u while the lowest estimated age is 2,59 years at peak load that is 100% with transformer lifespan 6,775 p.u