This paper deals with the synthesis and properties of novel oligodeoxynucleotide containing 6,3'-methanodeoxyuridine which is connected between C-6 of pyrimidine base and C-3' of deoxyribose by a methylene group. The modified base was successfully introduced into the oligodeoxynucleotide by the standard method. The modified oligodeoxynucleotide formed the less stable duplex with both complementary DNA and RNA. The CD spectra indicated that the duplex formed the noncanonical duplex structure distinct from the A-form or B-form structure.