The energy-dispersive X-ray diffraction measurements using synchrotron radiation at SPring-8 for expanded fluid Hg were carried out for the first time in the density range from the liquid to the dense vapour region including the metal–nonmetal (M–NM) transition region. Densities ranged from 13.6 to 1.9 g cm-3. We obtained the structure factor, S(k), and the pair distribution functions, g(r). The density variations of the obtained interatomic distance r1 and coordination number N1 were discussed in relation to the M–NM transition in fluid Hg.