Through our research we develop an Expert System called Transformer Fault Detection and abbreviation Exformer, to help engineers and technical's in detecting and diagnosis of oiled power transformer faults before it going out of service. We also use Fuzzy Logic in ambiguous data cases about gas ratios in transformer oil, which require use of fuzzy rules in knowledge base of expert system. We also discuss basis of using Artificial Neural Networks and choose number of layers, number of neurons and suitable neural network for power transformers faults analysis and compare.