X-ray diffraction, small angle X-ray scattering and inelastic X-ray scattering measurements for fluid Hg in the wide density range up to the supercritical region have been carried out using synchrotron radiation at SPring-8. We obtained the static and dynamic structure factors as a function of temperature, pressure and density. Based on the results we found that there exist medium-range fluctuations accompanying the metal–non-metal transition in fluid Hg. The scale of the fluctuations was about 10 A in space and sub-picoseconds in time. We discuss the possibility that such structural instability induces the metal–non-metal transition in expanded fluid Hg.