Diagnostic testing of power transformers plays an important role in detection of implicit failures, avoiding the possibilities of increased failure level or even breakdown for the most expensive equipment in power systems, which affects the operation reliability. Currently advanced diagnostic testing techniques such as Frequency Response Analysis, Dielectric Frequency Response etc. have been applied in addition with traditional ones but the efficiency is not as expected. In order to improve the diagnostic quality and the application feasibility of advanced techniques, this paper proposes a new method in determining main parameters in an equivalent circuit of several typical 110-kV power transformers based on the Frequency Response Analysis technique. The results show that the proposed method could be applied in improving the diagnostic quality for current failures of investigated power transformers.