The driving frequencies of electric vehicle motors and motors found in computer equipment are increasing due to the downsizing of the motors. This paper describes the magnetic properties of thin gauge electrical steel sheets and high Si steel steels for high frequency motors. And motor core losses are sometimes different from material core losses, which are measured by the Epstein test method. The change in loss is due to a number of known factors; the presence of plastic strain close to punching edge of manufactured motor sheets, assembling strains on the core material, the rotational exciting field and driving temperature etc. An understanding of these effects is of the utmost importance in the designing of efficient motors. In this paper these effects on magnetic properties of Non-oriented electrical steel sheets are explained.