We report a first-principles local density approximation band structure calculation on the electronic structure of high-TC superconductor MgB2.It is revealed that the band structure of magnesium diboride is of metallic type with a wide valence band.The density of states of electrons at Fermi energy is only about 0.72 (states/eV).We conclude that the present calculation excludes the possibility of BCS mechanism in MgB2.