A supercritical fluid is generally a solvent at atemperature above its critical temperature at which the fluid remains as a single phase regardless of pressure. The supercritical synthesis involves a liquid solvent which is completely miscible with the supercritical fluid to dissolve the solute. At this process condition the solute is insoluble in the supercritical fluid and the extract leads to instantaneous precipitation of the solute, resulting in the formation of nanoparticles and the methods using supercritical fluids are also powerful for the synthesis of nanoparticles. For these methods, the properties of a supercritical fluid (fluid forced into supercritical state by regulating its temperature and its pressure) are used to form nanoparticles by a rapid expansion of a supercritical solution. Supercritical fluid method is currently developed at the pilot scale in a continuous process. At high temperatures, aqueous solutions of metal salts rapidly hydrolyse to form nanoparticulate oxides. In the case of water, when it is heated towards its critical point (Tc 374oC, Pc 218 atm.), it undergoes a transformation considerably more dramatic than that of most other substances. It changes from the familiar polar liquid to an almost non-polar fluid. The change occurs over a relatively wide temperature range and the fluid becomes miscible both with organics and with gases. Also increasing the temperature renders water increasingly acidic and favours ionic processes over radical or purely thermal pathways.