Rheum palmatum, Chinese traditional herb, exhibits a great variety of anti-cancer and anti-viruses properties. This study rates antiviral activity of R. palmatum extracts and its components against Japanese encephalitis virus (JEV) in vitro. Methanol extract of R. palmatum contained higher levels of aloe emodin, chrysophanol, rhein, emodin and physcion than water extract. Methanol extract ($IC_{50}=15.04{mu}g/ml$) exhibited more potent inhibitory effects on JEV plaque reduction than water extract ($IC_{50}=51.41{mu}g/ml$). Meanwhile, $IC_{50}$ values determined by plaque reduction assay were $15.82{mu}g/ml$ for chrysophanol and $17.39{mu}g/ml$ for aloe-emodin, respectively. Virucidal activity of agents correlated with anti-JEV activity, while virucidal $IC_{50}$ values were $7.58{mu}g/ml$ for methanol extract, $17.36{mu}g/ml$ for water extract, $0.75{mu}g/ml$ for chrysophanol and $0.46{mu}g/ml$ for aloe-emodin, respectively. In addition, $10{mu}g/ml$ of extract, chrysophanol or aloe emodin caused 90 % inhibition of JEV yields in cells and significantly activated gamma activated sequence-driven promoters. Hence, methanol extract of R. palmatum and chrysophanol with high therapeutic index might be useful for development of antiviral agents against JEV.