Supercritical hydrothermal synthesis is the methodology of synthesizing metal oxide nanoparticles through hydrothermal reactions using a continuous flow reactor. Organic compounds and water can form a homogeneous phase at high temperature compressed conditions, which enables the in situ organic modification at the same time as the particle formation. This technique can be applied to various metal oxides. Continuous flow synthesis is advantageous for scaling up as well as the precise control of the products. In this study, fine control of ultrasmall metal oxides (CeO2, TiO2, ZrO2 etc) particles (e.g., < 5 nm) was achieved for metal oxides, which was difficult to synthesize previously. Recently, various synchrotron technique such as XRD, XAS, XES was utilized to precisely characterize ultrasmall particles in terms of particle size, crystal structure, local structure, and chemical state. Extraordinary properties and applications such as nanocatalysts, CO2 absorbent, ferromagnetic, luminescence, and nanofluids are expected for the ultrasmall metal oxides. In this presentation, oxygen carrier property is particularly focused on, which enables low temperature chemical looping processes.