Iron core of transformer somehow always have non-linear hysteresis characteristics. Generally, they are symmetry on their three legs of iron core. However, it may also suffer from unsymmetrical flux hysteresis characteristic due to inappropriate or special design. On the other hand, transformer is very sensitive to ferroresonance if it works on saturation area of its flux characteristic. Therefore, the effect of unsymmetrical flux distribution in three phases 150/20 kV power transformer on ferroresonance phenomenon was studied in this paper. Ferroresonance characteristics of symmetrical flux hysteresis of transformer were simulated using ATP/EMTP and the results were compared with the one of unsymmetrical. The result showed that at symmetrical condition, the overvoltage due to ferroresonance reached 389.81 kV on primary side and 23.54 kV on secondary one. Current response showed that current on primary and secondary sides increased until 1516.6 A and 2040.1 A, respectively. Under unsymmetrical condition the overvoltage became higher of about 2.27% from symmetrical condition. Different characteristics of ferroresonance were found on the transformer with smaller flux hysteresis because one leg was already work on its saturation area.