We developed cobalt-containing Fe₃O₄ particles, (Co, Fe)₃O₄, whose particle sizes and magnetic properties were precisely controlled to apply hysteresis-loss heating materials in thermoablation. The crcivity of the (Co, Fe)₃O₄ particles was adjusted in the range of 19.1 to 42.1 kA/m by changing the content of Co, while the particle size was maintained about 20nm. SiO₂-coated (Co, Fe)₃O₄ particles were dispersed in water. The temperature increase characteristics were examined for the dispersant containing (Co, Fe)₃O₄ particles with the crcivity of 24.0kA/m by applying a 120kHz ac magnetic field with an amplitude up to 50.1kA/m. The temperature increase ΔT/Δt was proportional to the amplitude of magnetic field, but was not saturated even when a magnetic field of 50.1kA/m was applied. The specific loss power (SLP) calculated from the ΔT/Δt and the particle content of the dispersant did not show a clear dependence on the coercivity, but a maximum value of over 1000W/g was obtained for a dispersant containing (Co, Fe)₃O₄ particles with a coercivity of 24.0kA/m under an ac magnetic field of 50.1kA/m.