In the course of our search for anti-Herpes simplex virus type I (HSV 1) substances from natural sources, we screened crude drugs for their antiviral activity using SRB assay. T he methanol extract from herb of Brassica rapa (Cruciferae) was found to inhibit HSV-1. Though bioassay-directed fractionation of the extract, anti-HSV-1 agent was isolated by chromatographic separation using Amberlite XAD-4 and Sephadex LH-20. The structure of compound I was elucidated by spectral means including $^1H-^1H$ COSY, HMQC and HMBC to be isorhamnetin 3-O-${eta}$-D-glucopyranoside (compound I). Compound I was active against HSV-1 with the 50% effective concentration of O.42mg/ml and the 50% cytotoxicity of 5.0mg/ml.