The Compton profile of the Ba doped silicon clathrate (Ba 8Si46) has been studied using the high-resolution Compton scattering technique. The Compton profile is sensitive to the change of wave functions, and the good agreement between experiment and theory validates a theoretical prediction. A difference Compton profile between the Ba doped and nondoped clathrates (Si136) has been experimentally obtained and compared with that of a first-principles band structure calculation. The experiment and calculation show excellent agreement with respect to the overall shape in the profile. Analyses of partial density of the states (DOS) predict that, by doping Ba atoms into the Si cages, Ba 6s electrons are transferred into Ba 5d orbitals that are strongly hybridized with Si 3p orbitals. The hybridized states form a sharp peak of the DOS in close vicinity of the Fermi level, which plays an important role for the occurrence of superconductivity in Ba 8Si46.