The transformer is one of the most common and most important electric machines for power companies. Cost of investment and the economic worth of a transformer are counted in millions of SEK. Because of that, proper maintenance actions that may lead to longer technical life time should be the aim in all situations. The aim with the report was to perform transformer diagnosis by dissolved gas analysis on Umeå Energi AB´s power transformers and a risk assessment. The report should also contribute to longer life time for the transformers. Oil samples were taken from each transformer two times. The first sample in summer 2013 and the second in April 2014. The gas concentrations in the samples was measured by an portable gas analyzing monitor. The samples were analyzed regarding to gas concentrations and the gas rate of change. The analyze was based on scientific methods like the total amount of dissolved combustible gases, Duvals triangle and Rogers ratios. The risk assessment was made as an internal comparison between Umeå Energi´s transformers. The results of the study showed large gas concentrations in several transformers in both times of sampling. The high concentrations appeared in old as well as in newer transformers. Therefore can the presence of gases not only be explained by natural aging, and different fault types can be suspected in some of the transformers. Duval´s triangle and Rogers ratios indicated various fault types. One of the transformers was immediately taken out of service after the first sample because of high level of a separate gas and the discussion about what actions to take was ongoing. A number of other transformers were taken into further consideration. Conclusions of the report were that transformer diagnosis is a very complex science where it is important to collect as much adequate information as possible to be able to make a correct analyze for every single transformer and to take proper actions. To be able to secure fault free operation of the power grid and to minimize the risk of wasting economic and material resources, and from a personal safety point of view, the power transformers must be maintained and tested regularly and be based on scientific principles. Faults in power transformers can lead to large impact on the environment and large effect on society if electricity is not available.