Abstract: A number of Compositousplants belonging to the genera Sonchus, Laetuca and Ixeris are used in Chinese medicine as Bai-jiang-cao in northern China. Among them, Sonchus arvensis L., called Bei-bai-jiang and included in the Chinese Pharmacopoeia 1977 edition under the name Herba Sonchi Arvensis, is most widely used.In this paper, the morphological and histological characters of the drugs derived from genera Sonchus, Lactuca, and Ixeris, viz. Sonchus arvensis L., S. oleraceus L., S. asper (L.) Hill., Lactuca tatariea (L.) C. A. Mey., Ixeris chinensis (Thunb.) Nakai, Ⅰ. sonchifolia Hance and Ⅰ. denticulata (Houtt.) Stebb., are described, compared, and illustrated with line drawings. The results showed that the medullary bundle, appearing in the stem-pith of the above drugs derived from genera Sonchus, Lactuca and Ixeris, is of great significance for the systematics of the respective genera in Compositae and for the identification of the related crude drugs. The microscopical similarities and differences found among the genera Ixeris, Sonchus and Lactuca, through the microscopical investigation of the seven Compositous plants are summarized. The formation of perimedullary sclerenchyma in the stems of the above species is discussed. Keys for morphological and microscopical identification of the above plant identification and for seven drugs derived from Compositae and Herba Thlaspi derived from Thlaspi arvensis (Cruciferae) are also given.