Extremely fine particles of needle-like lepidocrocite (gamma-FeOOH) were synthesized by the oxidation of aqueous suspensions of ferrous hydroxide using a bubble column with draft tube at a constant temperature of 30degreesC, and the effects of the reaction conditions or the oxidation rate were investigated in order to, determine the parameters that control the particle, size., When the concentration of oxygen in the -feed stream was varied under a constant gas velocity, the mean size based on the major axis of a needle-like particle decreased from 0.7 mum to 0.4 mum with increasing oxidation rate. By adding of NaH2PO4 to an aqueous Fe(OH)(2) suspension, in concentrations up to 1.01 mol/m(3) during the air oxidation and up to 0.9 mol/m(3) during the oxidation with 30% and 50% O-2, the major axis could be reduced to ca. 0.3 mum with the minor axis and the oxidation rate remained, almost unchanged.