Conditions for synthesizing acicular fine particles of goethite with high aspect ratio were devised by oxidizing mixed slurry comprising ferrous carbonate and hydroxide after neutralizing by alkali carbonate. By bubbling N-2 after neutralization into the slurry containing FeCO3 and Fe(OH)(2), dissolved CO2 is ejected. As a consequence, the relative concentration of CO32- decreases and that of hydroxide increases. These favor the increase of the aspect ratio of the goethite particles via two factors, i.e., (1) decrease in the critical size of the nuclei and (2) suppression of growth inhibition at the tip of needle-like particles due to CO32- adsorption. We propose a reaction scheme for the entire processing.