Magnetite with an aspect ratio of 10:1 was prepared by a seeding growth approach with a 0.35 T magnetic field applied. Studies show that magnetic field induced self-assembly of 50 nm diameter spherical particles, leading to the formation of a chain-like structure in alkaline media or a rod-like structure in acidic media. The addition of a small amount of seeds increases the yield of rod-like magnetite particles. The seeds are prepared by aging ferrous and ferric solution with sodium hydroxide as a precipitator. The aspect ratio of the rod increases with a decrease ratio of seed to iron salt. Seeds accelerate the initial reaction. A dependence of the morphology on the size of the seed is observed. Magnetic measurements reveal that magnetite rods are ferromagnetic with a saturation magnetization of 75.7 emu/g and coercivity of 386 Oe. Enhanced coercivity compared to without seeding is explained in terms of an increase of shape anisotropy.