Theoretical conformational analysis was carried out for the acyclic and cyclic tetrapeptides Ac-Cys-Pro-Gly-Cys-NHMe using ECEPP and optimization procedure for investigating the conformational preference of peptides having disulfide linkage. Calculated results indicate that cyclic Ac-Cys-Pro-Gly-Cys-NHMe forms compactly fold conformations with type II beta-bend at the Pro-Gly portion, and also show fairly good agreement with experimental results of the NMR spectroscopy for the tetrapeptides having Cys-Pro-Gly-Cys sequence.