6-Gingerol, a major pungent component of ginger (Zingiber officinale Roscoe, Zingiberaceae), has been reported to have antitumor activities. However, the metabolic fate of 6-gingerol and the contribution of its metabolites to the observed activities are still unclear. In the present study, we investigated the biotransformation of 6-gingerol in different cancer cells and in mice, purified and identified the major metabolites from human lung cancer cells, and determined the effects of the major metabolites on the proliferation of human cancer cells. Our results show that 6-gingerol is extensively metabolized in H-1299 human lung cancer cells, CL-13 mouse lung cancer cells, HCT-116 and HT-29 human colon cancer cells, and in mice. The two major metabolites in H-1299 cells were purified and identified as (3R,5S)-6-gingerdiol (M1) and (3S,5S)-6-gingerdiol (M2) based on the analysis of their 1D and 2D NMR data. Both metabolites induced cytotoxicity in cancer cells after 24 h, with M1 having a comparable effect to 6-gingerol in H-1299 cells.