With the volume expansion by heating up toward the critical point

typical liquid metal mercury undergoes metal-nonmetal transition (M-NM) at a density around 9 g/cm3. To study the structure changes of fluid Hg during volume expansion

we have carried out X-ray diffraction measurements for expanded fluid mercury in a wide density region from liquid to dense vapour region using synchrotron radiation at SPring-8. We have succeeded in developing a new high-pressure vessel

up to 1700C under 2000 bar and with 7 scattering windows for energy-dispersive X-ray diffraction (EDXD) measurements under high temperature and high pressure. It was found that the reliability of the structure factors

S(k)

and the accuracy of the pair distribution functions

g(r)

are much better. Reliable relations of the coordination number and the correlation distance with the density of fluid Hg were obtained. Structural model of volume expansion of fluid Hg is proposed based on our new results. Structural changes with decreasing density are discussed in relation to the M-NM transition in fluid Hg.