Supercritical fluids have shown an increasing interest as reactive media (tunable properties from liquid to gas) to synthesize nanostructured materials by thermal decomposition of inorganic precursors at relatively low pressure and temperature. The particle formation process (nucleation and growth) is made by high supersaturation in the supercritical fluid. So, the adjustment of synthesis process parameters results in a precise control of particle shape, size (between 10 nm and 10 μm), and chemical composition. We present the technique of thermal, chemical-heat, and structural strength treatment of materials to produce nanostructured nickel oxinitride in supercritical ammonia (solvent and reactant) from the thermal decomposition of nickel hexafluroroacetylacetonate (280°C, 18 MPa). A preliminary study concerning magnetic properties of the material was done and a correlation between particle size and magnetic behavior was pointed out.