Magnetic properties of a motor core are affected by the distortion due the cutting and the compression caused by a shrink fitting etc. The report which investigates such behavior systematically is few, although the examination of the detailed behavior of magnetic properties is important. In this paper, the mechanical stress due to the shrink fitting is examined and the mechanical stress of an actual stator core is measured by using strain gauges. Iron losses of actual motor cores made of wire cut and punching, and with and without shrink fitting are compared with each other. It is shown that a considerably large compressive mechanical stress is applied by the shrink fitting and the measured mechanical stress is similar to the analytical one. The iron losses of motor processed by wire cut and press become similar values when the compressive force due to the shrink fitting is large.