Frequency response analysis (FRA) is a usual method for diagnosing winding, core, insolation faults in transformer. By the change of the FRA traces, the faults can be detected. This study presents the interpretation on influence of untested winding's fault. A special power transformer of rating 10 kV/400 V, three-phase, 50 Hz, YNyn0 has been developed for carrying out FRA tests by practically simulated various winding faults in different phase. The windings of phase ‘A’ and phase ‘B’ on HV side are tested with different types of faults, respectively. The results of these experiments are displayed and discussed. It shows that when the untested winding is shorted, the influence of the inductive fault can be eliminated.