Five new ellagitannins, platycaryanins A (27), B (26), C (30), and D (25), and platycariin (31), and a new complex tannin, strobilanin (32), have been isolated from the fruits and bark of Platycarya strobilacea SIEB. et ZUCC. (Juglandaceae), together with twenty-six known compounds. On the basis of spectroscopic and chemical evidence, platycaryanins A, B and C, and platycariin have been characterized as the ellagitannins having a tergalloyl ester group, while platycaryanin D was found to be an unusual ellagitannin possessing (R)-hexahydroxydiphenoyl esters at the glucopyranose 2, 3- and 4, 6-positions. During these chemical studies, re-examination of the similar ellagitannins, alnusnins A and B, which were previously reported from Alnus sieboldiana, led us to revise their structures to 29 and 28, respectively, in which the tergalloyl group forms a lactone ring. Furthermore, biomimetic conversion of ellagitannins having a glucopyranose core into C-glycosidic ellagitannins was successfully achieved for the first time, and the reaction was applied to the structure elucidation of platycariin (31).