The results of ab initio HF/SCF calculations on nine low-energy conformers of N-acetyl-N’-methylprolineamide (Ac-Pro-NHMe) with trans/cis imide bonds and down/up puckerings using the STO-3G, 3-21G, and 6-31G** basis sets are reported. Single-point MP2/6-31G** calculations were performed on the HF/6-31G** optimized conformations. In ab initio calculations at all levels, the intramolecularly hydrogen-bonded C-7 conformer is found to be most preferred, which is consistent with the structures deduced from IR, CD, and NMR experiments in nonpolar solvents. From HF/3-21G to HF/6-31G**, the trend for the lowering of energies of Ac-Pro-NHMe appears to be quite similar to that found for alanine dipeptides, while the variations in backbone torsion angles phi and psi (av Delta phi approximate to 2 degrees and Delta psi approximate to 9 degrees) are different, which may be attributed to rigid constraints on the N-C-alpha rotation imposed by pyrrolidine ring closure. The MP2 corrections increase the HF/6-31G** energies by about 1 kcal/mol, except for the conformer tCu. The down puckering seems to be about 1.5 kcal/mol more stable than the up puckering at the HF/6-31G** and MP2/6-31G** levels. The HF/6-31G** optimized bond lengths and bond angles as well as endocyclic torsion angles chi’s of the pyrrolidine ring are, in general, consistent with the corresponding values of X-ray structures of peptides, but the strong dependence of structural parameters on the conformation cannot be seen. The preference and population for down and up puckered structures with the trans and cis imide bonds calculated at the HF/6-31G** and MP2/6-31G** levels agree reasonably with the results obtained from the analysis of X-ray structures of proteins. The degree of puckering for each conformer is described by three puckering amplitudes, and the Linear correlations between any two of them are discussed.