[en] We report the high-pressure behavior of plasmon in polycrystalline Li up to 15 GPa at room temperature studied by inelastic x-ray scattering and ab initio calculation. The plasmon energy () increases with decreasing atomic volume (), and the slope exhibits a discontinuity at bcc  →  fcc structural phase boundary reflecting the electronic band structure change. The plasmon peak width () versus momentum transfer (q) curve of bcc-Li below 6.5 GPa keeps similar parabola-like shape. Above 8.4 GPa, where Li is in fcc, it changes from that of bcc-Li and has a convex shape. (paper)