The interfacial Properties of ferrosoferric oxide suspended in aqueous KNO$_3$ solution are investigated by studying the zeta potentials and surface charge densities at $25\,^\circ\!C$. The zeta potentials are obtained by measuring the electrophoretic mobility and the surface charge densities by potentiometric titrations in the aqueous KNO$_3$ solutions of different concentrations from $10^{-3}$ to $10^{-1}$M. The data are interpreted by the surface dissociation and complexation model of Davis, et als. and the modified model.