Abstract: The kinetics of a-FeO(OH) formation in weak acidic medium was studied. The effects of initial concentration, temperature, pH value, partial presure of oxygen, air flow rate and agitating intensity on the oxidation rate of Fe(OH)_2 suspensions have been investigated. It is confirmed that reaction order for Fe(Ⅱ) is zero at the stage of a-FeO(OH) seeds formation, and around 0.5 at the stage of the seeds growth, and in both processes of the partial pressure of oxygen appears to be first order. The activation energy is determined respectively. Dissolution-oxidation equilibrium existing in the process of seeds formation has been assumed, by which the zero order reaction for Fe(II) and plateau in the pH-t curve may be explained. Physical process is the rate-determining step of the heterogeneous reaction. The studies of the dinetics of α-FeO(OH) formation may provide some fundamental information for the reactor design, the engineering scale-up and the optimizing control for the industrial process.