Relationship between thermal characteristics of permeability and microstructure of Ni-Zn ferrite were investigated. In non-stoichiometric (Fe2O3< 50 mol%) Ni-Zn ferrite, NiO phase and ZnO phase exist as secondary phases in addition to spinel phases which were confirmed to have a stoichiometric composition. Permeability is generally expressed in an equation μi= Is2/|K|, and in spinel ferrite the anisotropy K is a cubic anisotropy (Kc) originated in Fe3+. In this group of ferrite, however, the secondary phases give pressure to the spinel phases and the uniaxial anisotropy (Ku) generates by a partial strain of cubic system. So that, K is expressed in the sum of Kc and Ku. Thus, the change in the thermal characteristics of permeability can be explained in terms of the generation of secondary phases. At present, this model is quantitatively operational and is programed in the material design system for soft ferrites, MAGSYS-F.