Summary. -Herpes simplex virus 1 (HSV-1) is one of the most prevalent human pathogens in both industrialized and developing countries. This study was performed to analyze the antiviral activity of purified flavonoid from Polygonum perfoliatum l. against HSV-1 infection in vitro and in vivo. Flavonoid showed no inhibitory effect, when treated before virus infection, but it strongly inhibited viral replication and cell-to-cell spread which was vital for the virus's propagation. The therapeutic effect of the flavonoid in treating HSV-1 induced encephalitis was also investigated in mice. A dose-dependent increase of survival rate and mean survival time (MST) were observed in the flavonoid-treated mice. These results suggested that the flavonoid may be a viable therapeutic option for recurrent HSV-1 infection.Keywords: herpes simplex virus 1; flavonoid; Polygonum perfoliatum l.; antiviral, Hep-2 cell; mouse Acta virologica 58: 368 -373, 2014 doi:10.4149/av_2014_04_368 * Corresponding author. E-mail: zqyang@whu.edu.cn; phone: +862768759136. Abbreviations: ACV = acyclovir; EC 50 = 50% effective concentration; HSV-1= herpes simplex virus 1; IFA = immunofluorescence assay; MST = mean survival time; p.i. = post infection; SI = selectivity index; TC 50 = 50% toxic concentration