A non-linear loads on a transformer leads to increased no-load losses which cause increased operational costs and additional heating in transformer parts. It leads to early fatigue of insulation, premature failure and reduction of the useful life of the transformer. To prevent these problems, the rated capacity of transformer which supplies non-linear loads which generate harmonics, must be reduced. Standard transformers are normally designed and built for use at rated frequency, rated voltages and sinusoidal load current. In this paper is analyzed a standard 50 KVA three phase power transformer which supplies non-linear loads. The core losses (no-load losses) is evaluated using the three dimensional model of the transformer developed in Ansoft Maxwell based on valid model of transformer under high harmonic conditions. And finally a relation associated with core losses and amplitude of high harmonic order are reviewed &analyzed and then a comparison is being carried out on the results obtained by different excitation current in transformer windings.