Fe3O4 nanoparticles were functionally coated with polyethylenimine (PEI), which contained a high concentration of -NH2 groups, for the efficient harvesting of microalgae. The functional magnetic nanocomposites were 12 nm in diameter and 69.77 emu/g of saturation magnetization. Using a nanocomposite dosage of 20 mg/L for harvesting Chlorella ellipsoidea cells, a harvesting efficiency of 97% was achieved within 2 min. Increasing the temperature resulted in an increase in harvesting efficiency. The adsorption isotherm data fit the Langmuir model, suggesting that the adsorption was monolayer. The adsorption capacity of the Fe3O4-PEI nanocomposites for the microalgal cells reached up to 93.46 g-DCW/g-nanocomposites through electrostatic attraction and nanoscale interactions between the nanocomposites and the microalgal cells. The functional nanocomposites provide a base for efficient microalgae harvesting with clear advantages such as rapid execution, low energy consumption, and improves water-use in the algal harvesting process. (C) 2013 Elsevier B.V. All rights reserved.