Hydrophobic Mg(OH)(2) nanoparticles were successfully synthesized via an in situ surface modification method in a novel impinging stream-rotating packed bed (IS-RPB) reactor using oleic acid (C17H33COOH, OA) as a surface modifier, magnesium chloride hexahydrate in the presence of ethanol as a precursor, and sodium hydroxide as a precipitant. The products were characterized by Fourier transform infrared spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and thermogravimetry-differential scanning calorimetry (TG-DSC). Compatibility with organic solutions was determined by sedimentation tests. The prepared nanoparticles exhibited regular hexagonal lamella with an average diameter of 30 nm when OA is added to the reaction system; this result indicates that OA regulates the morphology of the Mg(OH)(2) nanoparticles. XRD revealed that the high-purityMg(OH)(2) product presents a brucite structure, and the I-001/I-101 of hydrophobic Mg(OH)(2) (0.86) was higher than that of the blank Mg(OH)(2) (0.63). FTIR analysis showed that OA bonded to the surface of the Mg(OH)(2). Compared with the blank Mg(OH)(2) product, the product obtained through the proposed method possesses excellent hydrophobic properties, including a high water contact angle of 101.4 degrees and good compatibility with liquid paraffin. TG-DSC analysis indicated that the total percentage of mass loss of hydrophobic Mg(OH)(2) (40.88%) was higher than that of the blank Mg(OH)(2) product (33.18%). The in situ surface modification method proposed in this work presents potential use in the large-scale production of Mg(OH)(2) nanoparticles. (C) 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.