The flexibility of succinimide molecule has been studied for the first time by quantum-chemical (at the MP2 level with up to the 6-311G(3df,2p) basis sets) and gas-phase electron diffraction (GED) methods using vibrational spectroscopy data from literature. The analysis of vibrational spectra, performed for the molecular model of C-2v symmetry (predicted by high-level ab initio calculations) using the scaling procedure, has shown that the two out-of-plane ring motions, that is, ring-bending and ring-twisting, are practically pure modes of different symmetry types and can be considered separately. The one-dimensional potential curves for the ring-bending and ring-twisting vibrations calculated at the MP2(full)/6-311G(3df,2p) level could be approximated by harmonic and anharmonic functions, respectively. The diverged energy levels for the ring-twisting vibration and the constant transition frequencies for the bending motion, obtained by the solution of the direct one-dimensional problem for the nonrigid model, demonstrate this statement. In the GED analysis, the succinimide molecule with a large-amplitude ring-twisting motion was described by a dynamic model with the distribution of pseudoconformers according to the calculated potential function taking into account structural relaxation effects from the MP2(full)/6-311G(3df,2p) calculations. This model greatly improved the fit of the GED intensities (R factor decreased from 4.6% for static model to 2.8%). The equilibrium molecular parameters re determined in the dynamic approximation are very close to the corresponding values from the ab initio calculations. At the same time, the parameters of the -CH2-CH2- fragment involved in the ring-twisting motion deviate considerably from those obtained for the static model (C-C bond length by 0.027 angstrom, =C-C-H. C-C-H, and H-C-C-N angles by up to 7 degrees). The flexibility influence on the C-C bond length is several times larger than the calculated vibrational correction (r(e) - r(a)) as well as the experimental uncertainty.