High pressure synthesis is a powerful technique in searching for new materials. Generally speaking, however, it used to be almost impossible to obtain single crystal samples of these high-pressure phases. To be reported here is the single crystal growths of various transition metal oxides by means of flux method based on the synchrotron X-ray powder diffraction studies at high pressures of several GPa. Some results on the structure analysis based on synchrotron X-ray powder diffraction will also be shown.