Three types of sugar modified pyrimido.4,5-b] indole nucleosides (2'-deoxy-2'-fluororibo-, 2'-deoxy-2'fluoroarabino- and arabinonucleosides) were synthesized by glycosylation of 4,6-dichloropyrimido.4,5b] indole followed by modification of sugar moiety and introduction of substituents into position 4 by cross-coupling reactions or nucleophilic substitutions. Some 2'-fluororibo- and 2'fluoroarabinonucleosides displayed interesting anti-HCV activities (IC50 = 1.6-20 mu M) and the latter compounds also some anti-dengue activities (IC50 = 10.8-40 mu M).