A new series of 3-allylseleno-6-alkylthiopyridazines 6a-6g was synthesized by two synthetic routes from 3,6-dichloropyridazine to develop new anticancer agents. These new compounds showed antiproliferative activities against breast cancer (MCF-7) cells in CCK-8 assays, and could be promising candidates for chemotherapy of carcinomas. Compound 6e (3-allylseleno-6-pentylthiopyridazine) showed higher potency than 5FU for inhibiting the growth of these cell lines. This suggests the potential anticancer activity of compound 6e.