Five new labdane diterpenes (1-5) together with seven known diterpenes have been isolated from a methanol extract of the aerial parts of Aster oharai. Five new structures were determined as 7alpha-hydroperoxylabda-8(17),14-dien-13(R)-ol-4-O-acetyl-alpha-L-6-deoxyidopyranoside (1), 7alpha-hydroperoxylabda-8,14-dien-13(R)-ol-4-O-acetyl-alpha-L-6-deoxyidopyranoside (2), labda-7,14-dien-13(R)-ol-4-O-acetyl-alpha-L-rhamnopyranoside (3), labda-7,14-dien-13(R)-ol-3-O-acetyl-alpha-L-rhamnopyranoside (4), and labda-7,14-dien-13(R)-ol-2-O-acetyl-alpha-L-rhamnopyranoside (5), on the basis of spectroscopic methods. Compound 1 showed moderate cytotoxicity against four cultured human tumor cell lines with ED(50) values ranging from 1.1 to 7.7 microg/mL.