In the reaction of Co modification of &ggr;‐Fe2O3, we found a very interesting phenomenon. When cations of Ca2+, Sr2+, or Ba2+belonging to IIA metals are adsorbed on the surface of &ggr;‐Fe2O3prior to Co2+ion adsorption, the coercivity of Co modified &ggr;‐Fe2O3is signifcantly increased compared to &ggr;‐Fe2O3modified only with Co2+. In contrast, when the Mg2+ion is adsorbed prior to Co2+ion adsorption, the coercivity is rarely increased. In this paper, we made the following hypotheses in order to interpret these phenomena: (1) Co2+ions are adsorbed both on the tetrahedral (A) site and the octahedral (B) site in the spinel structure of &ggr;‐Fe2O3. When Co2+ion occupies the B site and forms cobalt ferrite, the coercivity increases as a result of the surface magnetic anisotropy. (2) Because cations of alkaline earth metals (Ca,Sr,Ba) are easily adsorbed on the A site, adding alkaline earth metal before cobalt addition results in an increase in adsorption selectivity of Co2+ion on the B site. (3) On the other hand, Mg2+ion which is easily adsorbed on the B site prevents Co2+ion occupancy of the B site and as a result the coercivity does not increase.