Transformers illustrate the majority of capital investment in transmission and distribution centers. Additionally, interruptions of power transformer have a significant economic effects on the performance of an electrical network. For this reason, the safe and reliable performance of transformer, directly related to the security and stability of power system. This paper studies the growth of leakage flux and stray losses in the tank wall and core clamp of distribution transformer under unbalanced voltage condition. The computer-based simulation utilizing the three dimensional FEM that improved in Ansoft - Maxwell and surface impedances method as a tool for visualizing the leakage flux and stray losses on tank wall and core clamp of distribution transformers. FEM is one among popular numerical methods that is able to handle problem complexity in various forms.