In the diploma thesis we have focused on the influence of processing the grain oriented electrical steel on the magnetic losses of core. Purpose of the assignment was to evaluate influence of production processes on the losses in the transformer in the idle state. Core of the energetic transformers is made out of more than thousand sheets which need to be cut from supplied reels of metal sheet. Metal sheet is cut on the desired width with the machine for longitudinal cutting. Final shape of the sheets is made on the machine for cross cutting. Sheets are stacked into the core and finally upper yoke is closed. With the help of the device for measuring of specific losses we have analysed how individual phase of transformer core manufacturing, influences on the specific losses. On the basis of bigger quantity of samples, we have determined that the biggest influence on the losses has longitudinal cutting of metal sheet. Influence of the cross cutting procedure is almost negligible. On the basis of used samples for stacking procedure we have determined that this process has positive influence and slightly decrease specific losses.