In order to search for anti-Herpes simplex virus type 1(HSV-1) agents, we screened 80 specimens of Korean traditional medicine by SRB assay. The methanol extracts of Thujae orientalis Semen (Cupressaceae) showed strong anti-HSV activity among samples tested. From the butanol fraction of Thujae orientalis Semen anti-HSV-1 agent was isolated by chromatographic separation using Amberlite XAD-4 and Sephadex LH-20. The structure was elucidated by spectroscopic methods, and was identified as ${eta}-sitosterol$ (compound I). Compound I exhibited anti-HSV-1 activity with $EC_{50}$ of 0.6 mg/ml and $CC_{50}$ of 5.99 mg/ml, respectively.