Flavonoids are polyphenolic compounds that widely exist in plant kingdom, and the structure-activity relationship (SAR) of 25 flavonoids was studied on neuraminidase (NA) activity of influenza virus. Three typical influenza virus strains A/PR/8/34 (H1N1), A/Jinan/15/90 (H3N2), and B/Jiangshu/10/2003 were used as the source of NAs, the average of IC50s of these compounds on these NAs was used in the SAR analysis. The order of potency for NA inhibition was as follows: aurones > flavon(ol)es > isoflavones > flavanon(ol)es and flavan(ol)es. The SAR analysis of flavonoids on influenza virus NAs revealed that for good inhibitory effect, the 4′-OH, 7-OH, C4{double bond, long}O, and C2{double bond, long}C3 functionalities were essential, and the presence of a glycosylation group greatly reduced NA inhibition. The in vitro anti-viral activities of eight flavonoids were evaluated using a cytopathic effect (CPE) reduction method, the assay results confirmed the SAR as influenza virus neuraminidase inhibitors. The findings of this study provide important information for the exploitation and utilization of flavonoids as NA inhibitors for influenza treatment. © 2008.