These studies were conducted to discriminate Cynanchum wilfordii, C. auriculatum, Metaplexis japonica, and Polygonum multiflorum by morphological characters. Cynanchum wilfordii, C. auriculatum, and Metaplexis japonica(Asclepiadaceae) were easily distinguished from Polygonum multiflorum by twining stem to the left, producing opposite leaves, having laticiferous stems and leaves at cutting cross-sectionally, and lacking lignification of the stem. Four species had different morphological characters of flower. Comparing four species to fruit type, three species of Asclepiadaceae had follicle, while Polygonum multiflorum had achene with three wings. Follicle shape was lanceolate, narrowly lanceolate, and widely lanceolate in C. wilfordii, C. auriculatum, and Metaplexis japonica, respectively. Whereas seed color of four species was dark brown similarly, seed shape and 1,000 seed weight were useful characters to discrimate between three species of Asclepiadaceae and Polygonum multiflorum. Shape and color of root were thickened and yellowish brown in C. wilfordii and C. auriculatum. They were thin and long and white in Metaplexis japonica, and fusiform and reddish brown in Polygonum multiflorum. They would be useful characters to distinguish Cynanchum spp., Metaplexis japonica, and Polygonum multiflorum, however, impossible characters to discriminate between C. wilfordii and C. auriculatum.