Displacement of a hydroxyl group in pyrimidine nucleosides having a vicinal diol system by a fluorine atom was investigated by using diethylaminosulfur trifluoride (DAST). Though participation of the base moiety often thwarts the desired introduction of a fluorine atom, it was found that appropriate modification of the base and/or sugar moieties allowed the desired fluorodehydroxylation to occur, giving 5'-, 3'-β-, and 2'-α-fluorinated uracilnucleosides in good yields.