The diarylheptanoids (1-10) 1,7-bis-(3,4-dihydroxyphenyl)-heptane-3-O-beta-D-glucopyranosyl(1-->3)-beta-D-xylopyranoside (1), 1,7-bis-(3,4-dihydroxyphenyl)-heptane-3-O-beta-D-apiofuranosyl(1-->6)-beta-D-glucopyranoside (2), 1,7-bis-(3,4-dihydroxyphenyl)-heptane-5-O-beta-D-glucopyranoside (3), 1,7-bis-(3,4-dihydroxyphenyl)-5-hydroxyheptane (4), 1,7-bis-(3,4-dihydroxyphenyl)-heptane-3-one-5-O-beta-D-glucopyranoside (5), oregonin (6), hirsutanonol (7), hirsutenone (8), 1,7-bis-(3,4-dihydroxyphenyl)-5-hydroxyheptane-3-O-beta-D-xylopyranoside (9), and platyphylloside (10), isolated from the bark of Alnus japonica, were analyzed for their cytotoxic activities on various human and mouse cancer cell lines. The cytotoxic activities of these ten compounds were evaluated against murine B16 melanoma, human SNU-1 gastric cancer, human SNU-354 hepatoma cancer and human SNU-C4 colorectal cell lines. The diarylheptanoids showed potent cytotoxic activities against murine B16 melanoma cells and human SNU-C1 gastric cancer cell when the cell viability was analyzed by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide) assay.