Liquid Se

well-known as a typical semiconductor

undergoes semiconductor-to-metal (SC-M) transition at high temperature and pressure in the vicinity of the critical point. To understand such peculiar electronic and atomic structures of liquid Se

we have performed energy-dispersive X-ray diffraction measurements (EDXD) and small angle X-ray scattering (SAXS) using synchrotron radiation at SPring-8 for fluid Se

including liquid and dense vapour regions. The obtained structure factors

S(k)

and the pair distribution functions

g(r)

are with good accuracy by using a new high pressure vessel with 7 scattering windows. Besides confirming the preservation of twofold coordinated structure and the contraction of the nearest coordination distance in the metallic fluid region

it was found that the value of the first minimum in the g(r) increases obviously with increasing metallic properties

indicating the interaction enhancement of interchain in fluid Se. The SAXS experiment shows that the density fluctuation plays an important role in the formation of Se chain or Se dimer.