Domain structures in an applied field are simulated by minimizing the total energy in two dimensions. Wall positions and magnetization directions in the applied field are determined mainly by the balance of magnetostatic energyEsand Zeeman energyEm. On the other hand, the number of domains is determined by the balance of anisotropy energy and wall energy. Using this calculation, a soft magnetic film of low aspect ratio, such as a magnetic head core, is analyzed in the applied field. It was found that the energiesEsandEmare seemed to be determined by the film shape independently of the number of domains, and the ratio ofEstoEmis about 1 to −2. Moreover, the change in domain structure induced by applied field agrees with observations using a Kerr microscope. By analyzing a trapezoidal film which is similar to the thin‐film head core, we can obtain the angle of the film side surface whose domain structure is stable.