One hundred and twenty-two species of medicinal plants were tested for their inhibitory activity toward histidine decarboxylase (HDC), a histamine-synthesizing enzyme. The plant extract samples were prepared with 50% EtOH. Twenty-one extract samples exhibited significant HDC inhibitory activity. Among them, Artocarpus lakoocha, Amla, and Meadowsweet showed more than 90% inhibitory activity. The HDC inhibitory activities of the 21 extract samples were further examined by their taxonomic family categories and plant parts. Over 70% of the samples from the rose family exhibited inhibitory activity toward HDC, this rate being twice as high as that of other plant families. The samples obtained from the whole plant and from only the leaf, fruit, or seed showed higher HDC inhibitory activity than those taken from only the root or rhizome.