The particle size of magnesium di-hydroxide (MDH) was reported to have a significant impact on its flammability performance. Consequently, there have been many classical trials to produce ultra fine MDH for polymer flame retardancy. This paper reports on a novel continuous flow hydrothermal method for the instant synthesis of MDH nanoparticles. The hydrothermal conditions including: temperature, pressure, and flow rate were precisely controlled to achieve consistent product quality. MDH nanorods of 120 nm length and 20 nm diameters were reported by TEM. The tailored MDH crystalline phase and its phase transition with temperature (during its endothermic heat sink action) were investigated with XRD. The effectiveness of MDH as an endothermic heat sink material was evaluated by TGA and DSC. MDH was surface modified with organic polymeric surfactant in a continuous manner via post synthesis surface modification approach. Organic modified MDH exhibited complete change in surface properties and the nanoparticles were harvested from water phase to organic phase. Organic modified MDH exhibited smaller particle size with an increase in specific surface area and surface porosity compared with uncoated MDH. The synergism between MDH and an intumescent flame retardant (FR) agent commercially known as Exolit AP750 was investigated in order to achieve self extinguish multi-component epoxy nanocomposite at 10 wt% total solid loading. This is the first trial to synthesize and to functionalize ultra fine MDH continually; this approach might extend MDH usage as a clean and nontoxic FR agent that can be effectively surface modified. Therefore, enhanced flammability performance can be achieved at low solid loading level. (C) 2015 Elsevier B.V. All rights reserved.