From the nuts of Castanea mollissima Blume, a new kauranoid diterpene glycoside, named mollioside (1), was isolated. Its structure was established as (4R, 5S, 6R, 8R, 9S, 10S, 13R, 16R) 17-O-beta-D-glucopyranoside, ent-6,7-epoxy-6alpha-hydroxyl-6,7-secokaur-19-oic acid, 6, 19-lactone-16beta, 17-diol on the basis of HR-FAB-MS, 1D, 2D-NMR and CD spectral analysis. The aglycone (1a, named mollissin), also as a new compound, was obtained after enzymatic hydrolysis of 1. Both compounds exhibited significant growth inhibitory activity on HeLa tumor cells, but no activity on A375-S2.