Recent measurements of magnetic losses in a 3% Si‐Fe single crystal sheet ((100) [001] orientation) with a slab‐like domain structure have shown that the losses measured in the two ⟨100⟩ directions in the plane of the sheet are quite different. Using the Kerr magneto‐optical effect and high‐speed photography, the dynamic domain structures were observed. In the “hard” direction of magnetization, (magnetization in a ⟨100⟩ direction normal to the static domain walls) 90 degree domain walls are nucleated and annihilated each half cycle. The magnetic losses for magnetization in this direction were calculated by using a domain model for losses and considering the movement of 90 and 180 degree walls. A relationship between the losses in the hard direction and the “easy” direction (magnetization in a ⟨100⟩ direction parallel to the static domain walls) is found and is consistent with the previous experimental data.