In the formation of acicular and spindle-shape goethite (alpha-FeOOH) and acicular lepidcrocite (gamma-FeOOH) fine particles by air oxidation of aqueous suspensions of ferrous species, the particle size based on the major axis has been found to be proportional to the oxidation reaction rate raised to the power -0.30 similar to -0.44. This functional relationship was interpreted on the basis of the concept that the nucleation rate can be expressed as m-order in supersaturation of an intermediate species, polynuclear ferric hydroxo-complex.