Polyethyleneimine (PEI) functionalized Fe3O4 MNPs were synthesized by a modified hypothermal oxidative hydrolysis method.The magnetic nanoparticles showed positively charged surface,strong magnetic responsivity and uniform particle size distribution at 56.1±0.6 nm.Aggregation of these magnetic nanoparticles were observed on the surface of different type of bacteria.Magnetic capturing of bacteria were facilitated by these magnetic nanoparticles.The capturing efficiency could reach 90％ after two rounds of interactions of 5 minutes.The mechanism and process of interactions between bacteria and polyethyleneimine functionalized Fe3O4 magnetic nanoparticles were explored and discussed.The present study not only provides insight into interactions between Fe3O4@PEI MNPs and bacterial cells,but also opens a new avenue for designing and applying Fe3O4@PEI MNPs as biosensors in microbiology,medicine,and environmental science.