Acicular γ-Fe₂O₃ particles were treated at 90°C in alkali solution containing CO₂₊ and Fe₂₊. The coercivity of resultant particles remarkably increased with increasing CO₂₊/Fe₂₊ ratio, and reached a maximum value in the vicinity of 0.4 to 0.5. The increase of coercivity was attributed to the magnetic anisotropy of iron-cobalt ferrite which was considered to crystallize at the surface of γ-Fe₂O₃. The magnetic properties of γ-Fe₂O₃ particles coated by iron-cobalt ferrite, such as a variation of coercivity in annealing at 60°C or printing effect, were extremely stable compared with those of cobalt-substituted iron oxides.