An efficient approach for the synthesis of N-alkylbenzimidazole derivatives has been successfully developed via in situ reduction-cyclization of N-alkyl-o-nitroaniline and aldehyde in the presence of Na₂S₂O₄ under microwave irradiation. Two new N-propylbenzimidazole derivatives 5a-b and two new N-benzylbenzimidazole derivatives 5c-d were successfully synthesized in moderate yields (44-69%). In all cases, microwave method gave shorter running time and better yields. Compounds 5b-d possessed high toxicity against MFC-7 cell line in which compound 5c (EC₅₀ = 1.5439 mM) was found to be as active as the positive control Tamoxifen.