The PEG-coated Fe3O4 (PEG-Fe3O4) nanoparticles had been fabricated through chemical coprecipitationmethod. We comprehensively investigated the influence of different reaction conditions, such as vapor pressures with opening or sealing the beaker, molecular weights and amounts of PEG, on the structural and paramagnetic properties of PEG-Fe3O4 nanoparticles. The well-dispersed magnetic PEG-Fe3O4 nanoparticles with better size distribution can be obtained with adding 4 g PEG1000 while sealing the beaker. The possible growth mechanism was also discussed in detail. (C) 2014 Elsevier B. V. All rights reserved.