A series of novel γ-carboline ketones were designed, synthesized and evaluated for their cytotoxic activity in vitro against six human cancer cell lines (A549, SGC, HCT116, MCF-7, K562 and K562R). Biological evaluation revealed that almost all of the new compounds displayed moderate to potent cytotoxic activities against the tested cells. Among them, seven of the fourteen new compounds show more potent cytotoxic activities against K562R cell line than that of the positive control, taxol. Primary mechanism research on the most potent compound 6f indicated that it was a potent tubulin polymerization inhibitor, with IC(50) value of 4.3 μM, equivalent to that of CA-4, and arresting cell cycle in G(2)/M phase.