As an effort to search for new antiviral agents from traditional medicine, the hot water (HW) extract of twelve traditionally used medicinal plants in Taiwan was evaluated for their in vitro anti-herpes simplex viruses (HSV; including HSV-1 and HSV-2) and anti-adenoviruses (ADV; including ADV-3, ADV-8 and ADV-11) activities with a XTT-based colorimetric assay. Results showed that the tested HW extracts exhibited anti-HSV and anti-ADV activities at different magnitudes of potency. Among the twelve medicinal plants, Boussingaultia gracilis var. pseudobaselloides (Basellaceae) and Serissa japonica (Rubiaceae) possessed broad spectrum of antiviral activity. Ardisia squamulosa (Myrsinaceae) and Artemisai princeps var. orientalis (Compositae) were more effective in inhibiting ADV-8 replication than the other four viruses. Cell cytotoxic assay demonstrated that all tested HW extracts had CC₅₀ values higher than their EC₅₀ values. It was concluded that the twelve traditionally used medicinal plants in Taiwan possessed antiviral activity, and some of them merit further investigation.