3'-Deoxyuridine-5'-triphosphate (3'-dUTP) is synthesized starting from cordycepin in good yield. 3'-Deoxyuridine-5'-triphosphate strongly and competitively inhibits the incorporations of UTP into RNA by the RNA polymerases. 3'-Deoxyuridine-5'-triphosphate will be useful in studies at the molecular level on the relationship of template and substrate in RNA synthesis with chromatin, isolated nuclei or permeable cells, because it does not have any effect on poly (rA) synthesis[1].