Structures and vibrational force fields of all-trans-oligoenes of various chain lengths are studied by ab initio molecular orbital (MO) calculations at the second-order Moller-Plesset perturbation (MP?) level with the 3-21G and 6-31G* basis sets. Dependencies of the structure parameters and the force constants of trans-oligoenes on the chain length and the position in the chain are analyzed quantitatively. The structure and the force field of trans-polyacetylene are extrapolated from those of trans-oligoenes. Normal coordinate calculations are performed for trans-polyacetylene on this basis. The following results show the reliability of the structure and the vibrational force field of trans-polyacetylene obtained in the present study. (1) The calculated C=C and C-C bond lengths of trans-polyacetylene are in satisfactory agreement with the observed. (2) The calculated delta=0 (null phase difference) frequencies are in good agreement with the observed infrared and Raman frequencies. (3) The calculated phonon dispersion curves are consistent with some criteria obtained experimentally. (4) Most of the observed infrared bands which do not correspond to the delta=0 frequencies of trans-polyacetylene are assignable to peaks in the profile of the calculated density of vibrational states. (5) The calculated hydrogen-amplitude-weighted density of states is in reasonable agreement with the observed inelastic neutron scattering spectrum.