In magnetic materials used in motors, transformers, inductors, and so on, applied mechanical stress results in residual stress, which deteriorates the magnetic properties. Soft magnetic materials with zero magnetostriction have a distinct advantage in such applications. Silicon steel, in particular Fe-6.5 wt% Si, has attracted considerable attention as an excellent soft magnetic material with zero magnetostriction, low magnetic anisotropy, low iron loss, and high magnetic permeability, although there is some problem in productivity. In this chapter, we introduce the basic theoretical and computational methods to investigate these target systems in much detail and demonstrate how zero magnetostriction and low magnetic anisotropy at this particular composition of Fe-6.5 wt% Si can be explained theoretically or computationally. Here, the iron-rich region of the FeSi alloy at 1050 K is investigated by using first-principles phase field (FPPF) and special quasirandom structure (SQS) methods without relying on any experimental or empirical information. From the free energy comparison, we found that, for the Si concentration less than 25 at%, a solid-solution-like homogeneous phase is most stable, although a random pattern in nm scale consisting of B2 Fe4–xSix and D03 Fe3Si phases may appear at 6.5 wt% Si at somewhat lower temperatures. We made a conjecture that, around 6.5 wt% Si, such a random pattern in nm scale is the origin of the zero magnetostriction and low magnetic anisotropy.