A series of mononuclear ruthenium complexes containing monodentate ligand Benzimidazole have been synthesized and their biological propertieshave beenexamined. The ruthenium(II) complexes with substituted benzimidazole were prepared of the general formulae [RuCl3(HL)3H2O](HL1=2-(3-bromopropyl)-1H-benzo[d]imidazole; HL2=2-(4-chlorophenyl)-1H-benzo[d]imidazole;HL3 = 4-(1H-benzo[d]imidazole-2-yl)aniline; HL4=2-(4-nitrophenyl)-1H-benzo[d]imidazole; HL5=(1H-benzo[d]imidazole-2-yl)methanethiole). The complexes werecharacterized byconductivity measurements, elemental analysis, infrared, electronic, 1H and 13C-NMR and Mass spectral studies. The complexes were optimized by using Gaussion-09. The biological activities of ruthenium com-plexes were evaluated on human acute monocytic leukemia cells (THP-1) by MTT cell viability assay, ROS genera-tion and nuclear apoptosis analysis.The MTT assay revealed that these synthesized complexes, significantly reduces the viability of THP-1 cells, in a concentration-dependent manner. Interestingly, these compounds also produce the intracellular reactive oxygen species (ROS) as well as inducethe fragmented and condensed nuclei, suggest cell death by an apoptotic process. Biological activity data evidently supported that these complexes induce the apoptotic cell death in THP-1 cells.