A series of 6-arylamino-7-chloro-quinazoline-5,8-diones were prepared and evaluated for their in vitro cytotoxicity in cultured human cancer cell lines A549 (lung cancer), Col2 (colon cancer), and SNU-638 (stomach cancer). The preliminary structure-activity relationship has been described for providing further development of potent antitumor agents. To further investigate the cytotoxic mechanism, the effects of test compounds on DNA topoisomerase I and II activities have been assessed.