With　the　introduction　of　nano　zero　valent　iron　(nZVI)　technology　into　our　environment,　its　potential　environmental　risk　to　environmental　microorganisms　has　attracted　considerable　attention.　In　this　study,　Pseudomonas　putida　was　chosen　as　a　typical　strain　to　study　the　bacterial　toxicity　of　nZVI/Pd.　The　CFU　assay　results　indicated　that　nZVI/Pd　was　toxic　to　P. putida　cells　but　the　toxicity　decreased　with　an　increase　in　DO.　The　experiments　isolated　by　dialysis　bag　and　flow　cytometry　analysis　suggested　that　both　membrane　disruption　caused　by　direct　contact　and　oxidative　stress　were　the　main　bactericidal　mechanisms　under　the　aerobic　condition,　while　membrane　disruption　resulting　from　direct　contact　was　the　primary　bactericidal　mechanism　in　the　anaerobic　system.　Furthermore,　according　to　TEM,　SEM,　EDS,　XRD,　FTIR　and　XPS,　it　was　indicated　that　in　the　aerobic　system,　the　reactive　oxygen　species　(ROS)　generated　by　nZVI/Pd　could　oxidize　the　amide　and　hydroxyl　groups　into　carboxyl　groups,　resulting　in　a　decline　in　peptides　and　increase　in　polysaccharides.　In　addition,　the　ROS　also　accumulated　inside　the　cell　and　caused　cell　inactivation　via　oxidative　stress.　In　the　anaerobic　system,　the　adhered　nZVI/Pd　particles　would　attack　the　functional　groups　such　as　carboxyl,　ester　and　amide,　leading　to　the　decline　in　proteins　and　polysaccharides　and　subsequent　damage　of　the　membrane.　The　findings　provide　a　significant　guide　for　the　application　of　nano-bio　combined　technology.　©　2017　Elsevier　Ltd