A new class of N-substituted amidino-1-hydroxybenzimidazole derivatives (15-24) were synthesized and evaluated for their in vitro cytotoxic activities against human leukemia cell lines, HL-60 and K562. The preliminary results showed that compounds 16, 20, 21 and 23 had moderate antitumor activity against HL-60 cell line. Further investigation on the mechanism of the observed cytotoxic effects demonstrated that compound 21 increased the expression of autophagic and apoptotic genes and induced apoptosis of HL-60 cells.