New benzimidazole-4,7-diones substituted at 2-position were synthesized via a microwave-assisted reaction using 2-chloromethyl-1,5,6-trimethyl-1H-benzimidazole-4,7-dione 5b as a key intermediate compound. Their cytotoxicity has been evaluated on colon, breast and lung cancer cell lines. The dimer 17 was shown to possess excellent cytotoxicity comparable to that of mitomycin C.