To develop a new nucleoside analogue applicable to oligonucleotide therapeutics, we designed a 4'-thio analogue of an LNA/BNA monomer. Synthesis of 4'-hydroxymethyl-4'-thioribonucleoside was achieved by a tandem ring-contraction-aldol reaction of a 5-thiopyranose derivative and the subsequent Pummerer-type thioglycosylation reaction of the corresponding sulfoxide. Treatment of 4'-hydroxymethyl-4'-thiopyrimidine nucleosides with diphenyl carbonate in the presence of catalytic NaHCO3 gave the desired 4'-thioLNA/BNA monomers, which were introduced into oligonucleotides.