Three types of sugar modified pyrimido[4,5-<i>b</i>]indole nucleosides (2'-deoxy-2'-fluororibo-, 2'-deoxy-2'-fluoroarabino- and arabinonucleosides) were synthesized by glycosylation of 4,6-dichloropyrimido[4,5-<i>b</i>]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 (IC<sub>50</sub> = 1.6-20 μM) and the latter compounds also some anti-dengue activities (IC<sub>50</sub> = 10.8-40 μM).