Magnetic iron oxide (Fe3O4) nanoparticles (MNPs) was assigned to coat with amino-riched polyamidoamine (PAMAM) dendrimer (Fe3O4@PAIVIAM; Gn-dMNPs, n = 0, 1, 2, 3) and employed in the magnetic harvesting of oleaginous Chlorella sp. HQ due to the highly positive branched structure. The coating process was realized by stepwise grafting which encompasses Michael addition and amidation reaction. Amino-micromolecular amino acids were used to modify Fe3O4 MNPs as control groups to investigate the difference of microalgal harvesting between modification with polymer and micromolecule firstly. The results showed that the coating of highly positive branched PAMAM dendrimer could dramatically promote the adsorption between algae cells and nanoparticles, leading to a large drop of nanoparticles dosage, compared with amino acid (AA)-modified Fe3O4 MNPs (Fe3O4@AA). The influence of polymer coating thickness which is an unclear factor on harvesting was explored initially, suggesting that the harvesting efficiency was positively correlated with the coating thickness of PAMAM dendrimer and reached over 95% within 2 min when using 80 mg L-1 G3-dMNPs at pH 8.0. Moreover, the harvesting efficiency rose dramatically when pH decreased from 10.0 to 4.0 for G0 similar to G2-dMNPs, while the high efficiency for G3-dMNPs could be maintained in a wide pH range for the amino-riched branched structure. It can be attributed to increased electrostatic attraction and bridging flocculation caused by abundant active sites of the highly positive branched network on Fe3O4 MNPs. It can be expected that this novel magnetic flocculant has the potential to be applied to harvesting of oleaginous microalgae due to its high efficiency and cycling stability. (C) 2016 Elsevier B.V. All rights reserved.