Abstract: This study aims to isolate two flavonoids and two diterpenoids from Isodon eriocalyx(Dunn. )Hara and investigate their antiviral activities in vitro. Firstly, four compounds were isolated from the ethanol extract of I. eriocalyx by silica gel, ODS and RP-HPLC chromatography, and then identified by spectral data as isothymusin( 1 ), cirsimaritin( 2 ), coetsoidin A( 3 ), and maoecrystal D( 4 ). Next, in vitro antiviral activities against influenza(H1N1/H3N2)and respiratory syncytial virus(RSV)was investigated for the ethanol extract of I. eriocalyx and four compounds. Results showed that the ethanol extract of I. eriocalyx and four compounds exhibited inhibitory effects against influenza H1N1. Especially, compounds 1 and 2 showed better activities with EC₅₀ of(14. 45±4. 90)and(24. 54±3. 82)μmol/L. However, they showed weak inhibitory effects against influenza H3N2 and RSV. These results demonstrate that the ethanol extract of I. eriocalyx and compounds 1 - 4 have antiviral activity, especially against influenza H1N1. The flavonoids may be one kind of the effective antiviral substances. This study provides a scientific basis for the clinical application of I. eriocalyx.