The alkaloidal constituents of the seeds, roots, and tubers of Stephania cepharantha HAYATA (Menispermaceae), a folk medicine in China, have been studied, but those of the leaves have not been reported. We investigated the alkaloidal constituents of the leaves of this plant cultivated in Japan and obtained a new morphinane alkaloid, cephasugine (1), together with 14 known alkaloids. The structure of cephasugine (1) was elucidated as the N-methyl derivative of sinococuline (17) by comparison of spectroscopic data with those of cephakicine (16). This is the first report of the alkaloidal constituents of the leaves of S. cepharantha.