A series of 4-substituted colchicine derivatives were synthesized and evaluated with an eye toward developing new anticancer agents. As a result, 4-chlorocolchicine derivatives bearing a thioureide side chain at the C-7 position were found to exhibit significant cytotoxicities to three human cancer cell lines (A549, HT-29, and HCT116). In particular, compound 26 having an ethylthioureide group at the C-7 had high antitumor activity in vivo and a broad effective dosage range. Furthermore, compound 58, which has a (5-methylpyrazol-3-yl)thioureide group at the C-7 side chain, exhibited strong cytotoxicity and desirable metabolic stability in vitro.