Cinnamaldehydes have been shown to have inhibitory effects on farnesyl protein transferase (FPTase; EC 2.5.1.29) in vitro, angiogenesis, cell-cell adhesion, and tumor cell growth and to be immunomodulators. However, the mechanisms responsible for these effects remain unknown. To elucidate the molecular mechanism of the cinnamaldehyde derivative CB403 for growth inhibition, CB403 was synthesized from 2'-hydroxycinnamaldehyde. CB403-treated cells were weakly adherent to the culture dishes. In addition, CB403 inhibited tumor growth in these cells in a concentration-dependent manner. FACS analysis using human cancer cells treated with this compound showed cell cycle arrest in mitosis, which was correlated with a marked increase in the amount of cyclin B1. Furthermore, CB403 blocked in vivo growth of human colon and breast tumor xenografts without loss of body weight in nude mice. These results support the hypothesis that the cinnamaldehyde derivative CB403 exerts cytostatic properties by inducing mitotic arrest in cancer cells.