Recently, the use of nanomaterials as biostimulators for the methanogenic bacteria has been commonly deployed. This is to maximize the biogas production from livestock manure through the anaerobic digestion processes. Yet, the environmental impact of the nanomaterials as manure additives has not been evaluated. In this respect, different nanoparticles (NPs) of nickel (Ni), cobalt (Co), iron (Fe) and iron oxide (Fe3O4) were used in biogas production to study their environmental impact using life-cycle assessment (LCA) methodology. Global warming, greenhouse gas (GHG) emissions, acidification, eutrophication, resource depletion, human toxicity potential, and ozone layer depletion potential were investigated. The results showed that Co NPs was the most effective in reducing the greenhouse gas emissions through electricity production. The greenhouse gas emissions were 0.0366, 0.0276, 0.0225, 0.0336 and 0.0290 kg CO2 eq./MJ elect for the control, Ni NPs, Co NPs, Fe NPs and Fe3O4 NPs, respectively. Furthermore, Co NPs delivered the lowest acidification, human toxicity potential and eutrophication values. While, Ni NPs delivered the lowest resource and ozone layer depletion values. (C) 2019 Elsevier Ltd. All rights reserved.