Previously paper had demonstrated the effect of rheam palmatum against herpes simplex virus (HSV) in vitro(Luo J et al, J Natl Infect Dis, 1991). In order to confirm the therapeutic effect of rheum palmatum in herpes simplex virus infection in mice. Animial model of herpetic encephalitis was used to demonstrated the pontential of rheum palmatum. The mice were treated subcutaneously with rheum palmatum after 24 hour infection initiated. The dosage were 3.3g/kg. 6. 7g/kg and 11.3g/kg daily for 7 days. The results shown that the survival rate were 0.0%. 61.5% and 30.8% in each group, but means time death (MTD) were 6.3. 29.4 and 20. 5 days in each groups. The survival rate and MTD were 23.1% and 19.0 days in acyclovir group (100mg/kg daily for 7 days). All mice has no survival, MTD were 4.8 days in HSV control group. The mean virus titer of the rheum palmatum treated groups were Lower than in untreated group in brain, Liver, heart, These results indicate that the rheam palmatum has anti-herpes activity in vivo, antiviral effect is similar with acyclovir.