A novel cyclic dimer composed of chiral L-p-homoalanine was synthesized, and the formation of molecular assembly in water was investigated. The cyclic dimer was found to spontaneously form a spherical assembly with diameter of 40-100 nm in water. The cyclic dimer takes a boat conformation with cisamide configuration, and forms intermolecular hydrogen bondings in the assembly. The assembling property of the cyclic dimer of beta-amino acid is distinct from that of a-amino acid.