The ionization energy spectra and the spherically averaged momentum distributions of inner valence orbitals for diacetyl have been investigated by using a high resolution (ΔE=115eV, Δp≈01a.u.) electron momentum spectrometer. The impact energy was 1200eV plus binding energy (i.e. 1201—1240eV) and a symmetric non-coplanar kinematics was employed. The experimental ionization energy spectra have been compared with the published photoelectron spectra. The momentum profiles of inner valence orbitals 4agg+4buu have been compar ed with the theoretical calculations obtained using Hartree-Fock method and den sity functional theory．The measured momentum distribution is consistent with th e theoretical calculations.