A hydrothermal reactor for nanoparticle synthesis was designed and constructed for in-situ synchrotron radiation powder diffraction experiment at SPring-8 BL02B2 beamline. The oscillation measurement with a piston vibrator in the system contributed to measurement of homogeneous intensity distribution of Debye Scherrer ring. In-situ powder diffraction study of CeO2 nanoparticle synthesis was described to show the performance of the reactor. Three-dimensional diagram using pressure, temperature and size of CeO2 nanoparticles was determined by the system. The pressure and temperature dependence of particle formation speeds, lattice constants, and particle sizes was clearly determined as a reproducible result. A sensitive response of the cell volume by applying additional 7 MPa pressure was observed in the supercritical condition, whereas there were almost no volume changes in the liquid phase by applying additional 10 MPa pressure. The present in-situ system will be used to optimize the synthesis condition of functional nanoparticles.