PURPOSE: To obtain a magnetic-fluid with both good dispersibility and saturated magnetization retained, by dispersion, through solvent base oil substitution process, in a high-boiling point base oil, of surfactant-adsorbed ferrite fine particles having been dispersed in low-boiling point base oil.

CONSTITUTION: Surfactant-adsorbed ferrite ultrafine particles are dispersed in a low-boiling point base oil (pref. a hydrocarbon solvent with a boiling point ca. 30W110°C). Thence, the resulting dispersion is mixed with a high-boiling point base oil of a boiling point ca. 150°C compatible with said low-boiling point base oil (pref. higher alkyl-substituted naphthalene, dicarboxylic acid di-higher alkyl ester) followed by evaporation and elimination of said low-boiling point oil while irradiating supersonics to replace the solvent base oil, thus obtaining the objective magnetic fluid.