A series of cyclic peptides varying in ring size and in amino acid constituents was synthesized. Cyclization reactions of linear peptides were carried out by the azide, p-nitrophenyl ester, N-hydroxysuccinimide ester and diethyl phosphorocyanidate methods. Intermolecular cyclodimerization was also applied for the preparation of a cyclic hexapeptide. The reaction yield in each cyclization method was found to be sufficient for preparative purposes. Conformational analysis of the cyclic peptides was carried out by using proton nuclear magnetic resonance. The temperature dependency of peptide NH signals revealed that hexapeptides with the cyclo(-Gly-Xxx-Gly-)2 sequence are stabilized by intramolecular hydrogen bonding and are resistant to temperatureinduced conformational change.