Meso-DNAs, containing L-sugar and D-sugar in an alternate sequence as the backbone [Fig. 1, designated as LD-(dN)n], were prepared as a modificated oligonucleotides of natural- and enantio-DNA. The characteristics of meso-DNAs were analyzed, focusing on LD-(dA)n and LD-(dT)n. These L-sugar containing oligomers show resistancy to phosphodiesterases. Though both LD-(dA)n and LD-(dT)n bound their corresponding complementary homopolymers, apparent differences of the interaction mode between these two meso-DNAs were observed: [1] Concerning the complex formation with the complementary homopolymers, LD-(dA)n favors a triplex formation while LD-(dT)n forms only duplex, and [2] Concerning DNA/RNA selectivity for the complex formation, LD-(dA)n interacted with poly U more strongly than poly dT (RNA-selective), while LD-(dT)n showed stronger interaction with poly dA than poly A (DNA-selective). The similarities in CD-spectra of LD-oligomer/natural polymer complexes to natural complexes suggest that the conformation possesses a structure close to a natural right-handed helix conformation.