Magnetite chains of spheres with different radii and lengths were prepared by aging ferrous hydroxide gel with 0.2M KNO₃ under different external magnetic fields which induced the ultra-fine magnetite particles to combine into chain structure. The magnetite particle size and the chain length could be adjusted by varying the field intensity. The chains could be deposited on silicon substrates to form aligned structure in magnetic field. The hysteresis loops were measured in different directions by a vibrating sample magnetometer (VSM). The structure and magnetic properties were discussed.