Supercritical water (sc-H2O) reaction environment was evidenced with the capability to dramatically facilitate the solid-state reaction between hybrid oxides. This led to one-step and rapid (within several seconds) syntheses of ultrafine LaMnO3 nano-crystallites (at ca. 6nm) with no need of additional heat-treatment (literaturelly need to be heat-treated at 600–800°C for 4–6h in static air). The ultrafine LaMnO3 was shown with remarkable surface area and distinct oxygen mobility and reducibility, which yielded an activation energy (Ea) in toluene oxidation at only ca. 54.3kJ/mol, comparable to some noble metal catalysts.