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[4,5-b]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 μM) and the latter compounds also some anti-dengue activities (IC₅₀ = 10.8–40 μM).