On the basis of the non-covalent bond interactions in nucleic acids, we have synthesized a novel adenosine derivative tethering a phenyl group. Structure and thermal stability of the DNA duplexes bearing the adenosine derivative in the middle of a strand were investigated by CD spectra and the UV melting curves. We found that the conformation and the interactions of the duplexes were similar regardless of the nucleotide facing with the adenosine derivative in a duplex. This observation suggests the base pair-mimic geometry of the adenosine derivative accompanied with a conformational change of the nucleotide in the complementary strand.