Abstract: Magnetic Fe3O4 particles are ferromagnetic with a converse spinel crystal structure belonging to the cubic crystal system. Nano-magnetic Fe3O4 particles play an important role in the life sciences and biotechnology, and occur naturally in the living body. The function of magnetic particles in several kinds of living bodies is described from the viewpoint of crystal structure. The crystal configuration and magnetic domain structure of Fe3O4 nano-magnetic particles produced naturally in magnetotactic bacteria, human cerebrum and chiton are given.