Publisher Summary This chapter discusses the magnetic properties of nanocrystalline materials. The development of new soft magnetic materials with excellent characteristics of high magnetic flux density (Bs)of more than 1.5T combined with high μe of more than 105 at 1kHz was achieved by nanocrystallization of Fe—Zr—Nb—B—Cu amorphous alloys. The Cu-free quaternary Fe85.5Zr2Nb4B8.5 alloy which exhibits high Bs of 1.64T and high μe of 6.0 × 104 at 1kHz simultaneously, was also developed. In addition, it was found that the simultaneous addition of 1 at% P and 0.1 at% Cu to the Nb-poor Fe85Nb6B9 alloy results in good magnetic properties of high Bs of more than 1.6T and high μe of more than 4 × 104 at 1 kHz as well as good productivity. Furthermore, the dissolution of oxygen in the remaining amorphous phase caused significant improvement of high-frequency permeability of 103 in the frequency range 1 to 100 MHz by a drastic increase in ρRT to 103μΩm. Considering that these characteristics had not been previously reported, nanostructure control is concluded to be a useful method for the development of new types of high functional materials. The future progress of nanostructure-controlled materials is expected to enable the fabrication of new materials exhibiting useful characteristics which have not been achieved for conventional materials.