In this article, recent advances in the fundamental and applied researches on supercritical hydrothermal synthesis were reviewed. It was described how the supercritical condition is suitable for nanoparticle synthesis based on the nucleation theory with the unique features of metal oxide solubility and kinetics of the reaction around the critical points. Supercritical water also provides a good atmosphere to synthesize organic modified nanoparticles. The surface modification occurs during the nanoparticle synthesis, which leads to control the morphology of nanocrystals, namely the exposed surface. By this method, the most active surface of cerium oxide, (100) surface, could be exposed outside to increase the catalytic activity. The organic modification increases the affinity between nanoparticles and polymers to fabricate the hybrid materials with high loading of nanoparticles. Extremely high heat-conductive polymers could be fabricated by this method.