We developed cobalt-containing Fe₃O₄ particles whose particle sizes and magnetic properties were precisely controlled so that they could be used as hysteresis-loss heating materials for thermoablation in cancer therapy. The (Co, Fe)₃O₄ particles were synthesized through the formation of crystalline nuclei from an alkali solution containing Co²⁺, Fe²⁺, and Fe³⁺ ions, and then through crystalline growth using hydrothermal treatment. The coercivity was adjusted in the range from 150 to 490 Oe by changing the Co content. The saturation magnetization was in the range of 76-80 emu/g. The crystalline size obtained from the (311) plane by XRD was almost constant at about 18 nm. The particle size observed in TEM photos was about 20 nm, irrespective of the cobalt content. SiO₂ was coated on the surface of the (Co, Fe)₃O₄ particles to add dispersibility in a water medium. The dispersant of the SiO₂-coated (Co, Fe)₃O₄ particles showed excellent dispersibility, and no precipitation of particles was observed after the solution had been standing for one month.