Acid mine drainage is generally treated by neutralization process in which metal ions are precipitated as metal hydroxides, followed by settling of the precipitates. The treatment cost and the settling characteristics of the precipitates, such as settling velocity and sludge volume, depend on not only the concentration of metal ions, especially ferrous irons, but also the kind of neutralizing agent used and the introduction of various steps including oxidation, sludge recycle, etc. Neutralization process for mine drainage is outlined paying attention to the treatment cost and the sludge volume.