The effect of temperature, pH, the rate and time of oxidation, the concentration of ferrous ion in the starting suspensions as well as the amount of oxidant acting on the process of Fe(3)O(4) synthesis by Fe(OH)(2) suspensions are investigated. After 2h reaction at 90 degrees C under the oxidation of 10 g/L NH(4)NO(3), solution containing 0.25-0.35 mol/L iron(II) ion initially would yield the greatest amount of Fe(3)O(4), up to 95% Fe(3)O(4) could be formed. pH of the solution should be controlled between 9.0 and 11.0. X-ray diffraction (XRD) analysis shows that the product has spinel structure, which indicated that the product is Fe(3)O(4). Transmission electron microscopy (TEM) images show that the crystal size of ferrite is around 0.2 microm. The equilibrium composition of the synthesis reaction of Fe(3)O(4) is optimized by the minimization of the free energy of thermodynamics. It was found that the optimal condition for the synthesis of Fe(3)O(4) obtained through experiment is correspondent with that obtained through computer calculation. In the Fe(3)O(4) formation area given by Kiyama [M. Kiyama, Conditions for the formation of Fe(3)O(4) by the air oxidation of Fe(OH)(2) suspensions, Bull. Chem. Soc. Jpn. 47 (7) (1974) 1646-1650], the content of the product formed is not the same everywhere. The main factor that influences the content of the product is the amount of oxidant.