Ferromagnetic materials exhibit spontaneous magnetizations even without an applied magnetic field. Their internal structure contains regions called magnetic domains in which magnetic moments are aligned in the same direction. Magnetic moments in different domains point in different directions. For example, when the total magnetization in a ferromagnetic material is close to zero (magnetically neutral state or demagnetized state), the directions of the magnetic moments are determined so as to minimize magnetic energy of the ferromagnetic material. To understand the magnetic domain structure, the behaviors of magnetic moments and the magnetic energy of the ferromagnetic material need to be analyzed. Techniques in micromagnetics simulation are extremely effective for analyzing them. In this chapter, the basic terminologies concerning magnetic domains of ferromagnetic materials are introduced, techniques in micromagnetics simulation are explained, and calculation examples of the magnetic domain structures for the ferromagnetic materials are presented.