Time-resolved transmission electron microscopy (TEM) was used to follow the process of formation of the regular quadrangular prismatical beta-FeOOH single crystals. No phase transformation was found during the formation, though particle shape changed from acicular to spindle and from spindle to prismatical. The structure of the single crystals is identified as tetragonal from electron diffraction. A possible mechanism for formation of the single crystals was suggested : growth occurs by heterogeneous precipitation of Fe3+ ions on the surface of the homogeneously formed acicular beta-FeOOH nuclei. The homogeneous nucleation mechanism was confirmed by a instantaneous nucleation experiment. This interpretation is in accord with LaMer’s model.