With ab initio HF and density functional B3LYP methods along with the self-consistent reaction field method, the conformational structure on N-acetyl-N’-methylamides of thiazolidine residues (Ac-Thp-NHMe) is analyzed to find out the effects of the substitution of the CγH2 group in the prolyl ring by sulfur atom on the conformational preferences and puckering population in the gas phase and in solution (chloroform and water). The sulfur replacement of the CγH2 group of the prolyl ring changes the different backbone structures and prolyl puckerings from those of the Pro dipeptide in the gas phase and in solution. Although the populations of backbone conformations and puckerings are different in Thp dipeptide and proline dipeptides, as the solvent polarity increases, the population of conformations C with the C7 intramolecular hydrogen bond decreased, the population of PPII- or PPI- like conformations F conformation increased, and the cis populations increased. This results were similar for the Pro dipeptide. Whereas as the solvent polarity increases, the populations of the trans/up conformations decrease for Thp dipeptides but they increase for the Pro dipeptide.