A novel series of 1-anilino-5H-pyridazino[4,5-b]indoles was designed and synthesized in order to find novel potent anti-tumor compounds. Their structures were confirmed by MS, (1)H-NMR, and elemental analysis. All compounds were screened for their cytotoxic activity against two human cancer cell lines (Bel-7420, HT-1080). The compounds 8, 9, and 17 showed 50% growth inhibitory activity in low micromolar concentration (IC(50 )= 7.7 approximately 12.8 microM). Among them, compound 17 displayed the most potent anti-tumor activity with IC(50) values of 8.2 microM and 7.9 microM against Bel-7402 and HT-1080, respectively.