ABSTRACT:The preferred conformations of cyclic dipeptides were first studied systemically using the density functional theory (DFT) B3LYP method at the 6-31G(d) level. The structural characteristics of cyclic dipeptides were revealed, most of which have not been confirmed until now. Our studies showed that the six-member main circles of cyclic dipeptides composed of natural l-amino acid residues appeared as boat conformations. The important factors that influence conformations of cyclic dipeptides, such as molecular total energy, nuclear repulsion energy, molecular orbit, spatial effects, and reactive mechanism, are discussed in detail.