Five new nonalkaloid constituents, a neolignan, japonin A (1), a macrocyclic lactone, japonin B (2), a (phenylethyl)benzoquinone, japonin C (3), a phenanthraquinone, japonin D (4), as well as a dihydrostilbene, stilbostemin M (5), were isolated from the roots of Stemona japonica, together with eight known compounds. Their structures were established by spectroscopic analyses.