Abstract: Objective To investigate the effects of Alternanthera philoxeroides Griseb extracts against dengue virus in vitro. Methods MTT assay and observation of cytopathic effect (CPE) were carried out to determine the cytotoxicity of Alternanthera philoxeroides Griseb on C6/36 cell lines and its effects on dengue virus. Results None of the 4 kinds of Alternanthera philoxeroides Griseb extracts exhibited obvious cytotoxicity on the cells at different concentrations with the exception of that over 320 μg/ml. The 4 extracts all showed inhibitory effects on dengue virus. Statistical analysis of TD₅₀ and ED₅₀ by Probit regression method suggested that extracts from coumarin had the lowest toxicity on the cells (TD₅₀=535.91), whereas extracts from petroleum ether showed the strongest inhibitory effects on dengue virus (ED₅₀=47.43) among the 4 extracts. Conclusion Alternanthera philoxeroides Griseb possesses antiviral effects on dengue virus in vitro.