Magnetic properties of platelet Fe 3 O 4 particles were studied to apply them to magnetic hyperthermia or thermoablation using hysteresis-loss heating. As precursor materials, platelet α-FeOOH particles were synthesized by a hydrothermal treatment of the precipitated Fe 3+ ions in an alkaline solution of ethanolamine. Spinel-structured iron oxide particles were obtained from the platelet α-FeOOH particles by a liquid-phase reduction process using tetra-ethylene glycol as the reducing agent and were recognized as platelet particles with size 30–100nm by transmission electron microscopy. Their chemical composition and crystal structure were confirmed to be that of Fe 3 O 4 by X-ray diffraction and Mössbauer analyses. The coercive force and saturation magnetization of the Fe 3 O 4 particles were 11.9kA/m and 77.9Am 2 /kg, respectively. A tape was prepared by orientating the platelet Fe 3 O 4 particles in such a way that the particle planes were parallel to the tape plane. The calculated magnetic anisotropy field of the Fe 3 O 4 particles was about two times larger than the in-plane coercive force of 12.6kA/m of the oriented tape. The saturation field in direction perpendicular to the oriented tape was 390kA/m, which is close to the demagnetizing field in the platelet Fe 3 O 4 particles.