Transformers is a device on which cost effective supply of electricity mostly depends. Hence, to manage the life of transformers, to reduce failures and to extend the life of transformer, some measures are being adopted. Dissolved gas-in-oil analysis (DGA) is a common practice in transformer fault diagnosis. Some classical methods that depend on gases concentration in transformers oils are used to interpret transformer faults such as Dornenberg, Rogers, Duval triangle and key gases methods. These methods in some cases did not give the same results; therefore, an expertise method is developed to give the fault type according to the dissolved gases concentration in oil. A software code is designed using logic functions to get the type of the faults in transformers. Comparing the results from this software with the real laboratory cases as well as some cases in literatures is developed. The results explain the validation of software to detect the fault in transformer.