The invention relates to chemical technologies, specifically nanoparticles of flame retardant magnesium hydroxide, and a process for the preparation thereof. The present nanoparticles of flame retardant magnesium hydroxide, including surface-processed nanoparticles, have a hexagonal plate-like structure with a specific surface area of up to 20 m 2 /g, an average diameter of the secondary particles of up to 2 µm, a diameter of 10 % of the secondary particles of up to 0.8 µm, a diameter of 90 % of the secondary particles of up to 5 µm, with a longitudinal size of the primary particles of from 150 to 900 nm, and a thickness of from 15 to 150 nm. The present process for the preparation of nanoparticles of flame retardant magnesium hydroxide, including surface-processed nanoparticles, consists of two steps, wherein the first step consists in interacting an aqueous solution of magnesium chloride with an alkali component at a temperature of up to 100 °C and atmospheric pressure, with a molar ion ratio OH - :
