Crystal structures of DNA octamer with the sequence d(GCGAGAGC) have been determined by X-ray analyses to investigate the specific DNA structural motifs that are useful for designing various functional DNA molecules. The octamers are assembled to form an octaplex with G-quartets and water-mediated A-quartets. At relatively high potassium concentration, however, the octaplex is split into two quadruplexes, each of which contains two G-duets. The two crystal forms suggest a dynamic formation of an octaplex from two quadruplexes in solution.