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In this study, beta-FeOOH nanosolids were produced by compacting beta-FeOOH nanoparticles (average size is 12 nm) synthesized by the method of chemical hydrolysis. The X-ray diffraction measurements show that the structure and crystallite size of beta-FeOOH nanosolids do not change under high pressure up to 4.5 GPa, but differential thermal analysis indicates that the temperature of structural transformation increases strongly with increasing pressure, from 203.8 degrees C under atmosphere pressure to 274 degrees C under the pressure of 4.5 GPa. After treatment under hot pressure, the structure of the beta-FeOOH nanosolids changed significantly. We observed for the first time the transformation from the beta-FeOOH phase to the alpha-FeOOH phase under hot pressure of 3.0 GPa, 200 degrees C and 4.5 GPa, 350 degrees C and obtained a new metastable phase of FeOOH (tetragonal sys., a = 0.98968 nm, c = 1.02167 nm) under hot pressure of 4.5 GPa, 200 degrees C.