Water structure under high-pressure and high-temperature conditions has been revealed by X-ray diffraction and empirical potential structure refinement (EPSR) modeling. X-ray diffraction measurements of water at temperatures (298 K/30 MPa, 473 K/30 MPa and 573 K/30 MPa) and at high pressures (298 K/1 GPa, 473 K/0.35 GPa and 486 K/4 GPa) were performed on energy-dispersive type X-ray diffractometers in laboratory and in a synchrotron radiation facility, respectively. The EPSR modeling using the experimental diffraction data generated three dimensional structure of water including the position of hydrogen atoms. With decreasing density, the coordination number of water molecule decreases at high temperatures. Pressure bends the hydrogen bonds between water molecules and makes the water structure under high-pressure condition similar to that of simple liquid (Ar, Hg, etc.) The unique properties of water under extreme conditions (supercritical and high-pressure conditions) would originate from the change of hydrogen bonds between water molecules and the coordination number of water.