Insulating liquids have been applied to electrical insulation of transformers, circuit breakers and switch gears and have been investigated in different ways to enhance their performance. Dielectric liquids have been used for the transformer insulation and cooling applications since more than a century. Dielectric breakdown tests and the events and processes that affect the breakdown of the transformer insulation have been studied since last 40-50 years. Transformer oil can give a thorough picture of the internal transformer conditions. The fault diagnosis can also be done by investigating the transformer oil. For this reason, oil samples from a transformer that is out of service due to the internal faults are taken and tested to realize the reason behind the faults. Gas Chromatography Mass Spectrometry (GCMS) was employed to investigate the oil condition of the power transformer. GC-MS results show that high molecular weight substances in faulty oil are more than fresh oil. The results are compared with that of the fresh mineral oil supplied by the same manufacturer and presented in detail.