A chemical examination of the bark of Mallotus japonicus (THUNB.) MUELLER-ARG. (Euphorbiaceae) has led to the isolation of five new hydrolyzable tannins (16-20), together with fourteen known tannins (1-14). On the basis of chemical and spectroscopic evidence, the structures of compounds 16 and 17 were established as 1, 2-di-O-galloyl -3, 6-(R)-hexahydroxydiphenoyl-β-D-glucose and 1-O-digalloyl-3, 6-(R)-hexahydroxydiphenoyl-β-D-glucose, respectively, while compounds 18 (mallojaponin) and 19 (mallonin) were shown to be 1-O-galloy-2, 4-elaeocarpusinoyl-3, 6-(R)-valoneayl-β-D-glucose and 1-O-galloyl-2, 4-elaeocarpusinoyl-β-D-glucose. Compound 20 (mallotusinin) was characterized as a novel ellagitannin which possesses a unique 1, 1'-(3, 3', 4, 4'-tetrahydroxy)dibenzofurandicarbozyl group. On the other hand, examination of the leaves revealed the presence of hydrolyxable tannins (8-10, 12-15) all containing a β-D-glucopyranose core with ¹C₄-conformation. Furthermore, the orientation of the valoneayl group in mallotinic acid (13) and mallotusinic acid (14), which had remained unclarified, was determined on the basis of ¹H-¹³C shift correlation spectral analysis and chemical correlations.