Power 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 relate directly to the Security and stability of power system. This paper studies the growth of power losses and Derating of Distribution transformer, working under unbalance voltage and unbalanced load conditions. In this paper, computer-based simulation utilizing the two and three dimensional finite element methods (3-D and 2_D FEM) are exploited as a tool for visualizing magnetic fields of distribution transformers. Finite Element Method (FEM) is one among popular numerical methods that is able to handle problem complexity in various forms. The performance of the transformer when the load and supply are balanced compared to the case in which voltage or load is unbalanced indicates that the unbalanced load only causes the copper loss to increase, while the unbalanced voltage not only increases copper loss but also adds to the core loss. The indices of voltage unbalance factor (VUF) are used in this paper to evaluate the unbalance propagation. This process enables the software to simulate and analyze different electromagnetic parameters such as magnetic flux lines, flux density, losses, and etc, under different input sources with high accuracy.