The ionization energy spectrum and the spherically averaged momentum distribution of 1a′ inner valance orbital for cyclopentene 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. 1202—1234eV) and a symmet ric non-coplanar kinematics was employed. The experimental momentum profile of the 1a′ inner valence orbital is obtained and compared with the theoretical m omentum distributions calculated by using Hartree-Fock and density functional th eory methods with the basis sets STO-3G, 6-31G, 6-311++G** and au g-cc-pVTZ. The pole strength of the 1a′ inner valence orbital is estimated and the electron correlation effects are discussed.