Nanostructured ZrO2 was successfully prepared by using processing by pressurized hot water and pressurized hot/supercritical methanol in a flow regime. It was found out that processing temperature and amount of used water are crucial for ZrO2 crystallization. Processing by pressurized hot or supercritical methanol improved ZrO2 purity and affected microporosity, but did not cause ZrO2 crystallization. The effect of processing pressure on ZrO2 crystallization was also proved; at 10 and 30 MPa the crystallization of zirconia was enhanced accompanied with its improved purity. It was revealed that processing by both pressurized fluids at different conditions has significant impact on further crystallization of ZrO2 during post-thermal treatment, thus, the phase composition of ZrO2 can be tailored. While thermal treatment of nanocrystalline ZrO2 led to crystallization of bicrystalline ZrO2 mixture with major monoclinic phase, thermal treatment of amorphous ZrO2 resulted in crystallization of bicrystalline ZrO2 mixture with major tetragonal phase.