Porous composites of magnetite / hydroxyapatite with bimodal pore size distribution were prepared by hydrothermal method. Magnetite particles of about 1 µm in size with polyhedral shape were dispersed into hydroxyapatite porous matrix. The hydroxyapatite particles were rod-shaped crystals and the size of crystals were about 10 µm in length with the aspect ratio of over 25. Rod-shaped hydroxyapatite crystals were locked together to make micro-pores of about 0.8 µm in size. In addition, macro-pores of about 400 µm in size were prepared by our unique method. Hydroxyapatite in this composite was non-stoichiometric hydroxyapatite with calcium deficient composition. This composite must have the advantage of adsorptive activity and osteoconductivity, because the hydroxyapatite has many specific crystal surface and micro-pores. The composites of magnetite / hydroxyapatite must be suitable for the hyperthermia therapy of cancer in bones.