As an environmental nanotechnology, nano-sized materials have the potential to create novel and effective in-situ and ex-situ treatments for contaminated groundwater due to its high catalytic reactivity, large surface area, and dispersibility. In this study the efficiency of Cr(VI) reduction and immobilization using biotic and abiotic nano-sized materials (NSMs) and metal-reducing bacteria (MRB) was evaluated to remediate Cr(VI)-contaminated groundwater in batch and column tests. The results of this study revealed that the combination of the mixed MRB and bio-FeS/siderite performed the highest efficiency of Cr(VI) reduction and immobilization. Cr(VI) reduction by MRB and NSMs could impact on solubility of Cr(VI) and geochemical changes favorable for precipitation and adsorption.