Abstract: Nano-scale ferrofluids of PEG (polyethylene glycol)/Fe3O4were preparedviathe technology for synchronously preparing magnetic particles and covering surfactant and using water as the working fluid and PEG as the surfactant, and the effect of PEG content on the micromorphology and size of PEG/Fe3O4colloidal particles in ferrofluids was investigated. The results indicate that the micromorphology and size of colloidal particles are obviously influenced by PEG content in the reaction system, and products with a shape of sphere or bar can be prepared by changing PEG concentration. A further study shows that there exist the interactions of physical adsorption and hydrogen bonding between Fe3O4particles and PEG.