Abstract: It is well known that the research on nanometer solid has brought to people's great interest in recent years, but only a few works has been done so far about the properties of nanocrystalline under high pressure. In the present paper, we have synthesized a series of -FeOOH nanometer solid by treating the ultrafine powder under the pressure range of 0.0～4.5 GPa and the temperature range of 200～350 ℃. The ultrafine powder was synthesized by using chemical hydrolysis method and its average diameter is 12 nm. Using X-ray diffraction (XRD) and differential thermal analysis (TG-DTA), we investigated the structural transformation of these nanometer solids. The results of XRD and TG-DTA experiments show that under cold pressure of 0.0～4.5 GPa, the structure of the nanometer solids do not change considerably, but on the other hand, the temperature of the heat-induced phase transformation (from -FeOOH phase to -Fe2O3 phase) in these -FeOOH nanometer solids is raised obviously with increasing pressure, from 203.8 ℃ at normal pressure to 274 ℃ at 4.5 GPa. This variation can be interpreted by the effect of pressure on the coordinate state of the OH- group in the interface of nanometer solid. After treating under hot pressure, both the formation and temperature of the structural transformation of -FeOOH nanometer solid are very different from the results under cold pressure. Under the hot pressure of 4.5 GPa, the temperature of the structural transformation from -FeOOH phase to -Fe2O3 phase is far above 350 ℃. However under the condition of 3.0 GPa, 200 ℃ and 4.5 GPa, 350 ℃, we firstly observe the structural transformation, from -FeOOH phase to -FeOOH phase. It is more interesting that we obtain a new metastable phase of FeOOH in the above structural transformation at the condition of 4.5 GPa and 200 ℃. These special variation can be interpreted by considering the effect of pressure combining temperature on the OH- group in the nanometer solid at the same time.