Magnesium hydroxide [Mg(OH)(2)] nanoparticles were prepared using a novel impinging stream-rotating packed bed reactor (IS-RPB). Magnesium chloride and sodium hydroxide were used as the reactants. The effects of several synthesis parameters, such as the initial concentration of Mg2+, rotation speed, liquid flow rate, reaction temperature, and reactant concentration ratio, on the particle size, distribution, and morphology of Mg(OH)(2) were investigated. The liquid flow rate and rotation speed were determined to have a significant influence on the particle size and distribution of Mg(OH)(2) powder. The volume mean size decreased with an increase in the liquid flow rate and rotation speed. The size distribution for the lowest liquid flow rate was wider than that for the higher liquid flow rates. The morphology, crystal phase, and thermal behavior of the samples were also characterized through transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and thermal gravimetric analysis. The results indicated that Mg(OH)(2) nanoparticles prepared using the wet precipitation method in IS-RPB had small particle size and narrow size distribution. Moreover, the results indicated that the presented novel reactor (IS-RPB) might provide a new method for the production of nanoparticles in industrial applications.