  The spectral function of rho meson in hot/dense hadronic matter is studied by
taking into account the nucleon-loop on quantum hadrondynamics model level.
Different from the hot pion gas effect which changes the spectral function
slightly, the nucleon-antinucleon polarization (Dirac sea) makes the spectral
function very sharp and shifted towards the low invariant mass region
significantly due to the decreasing effective nucleon mass.
