Abstract: 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 m2/g, an average diameter of the secondary particles of up to 2 ?m, a diameter of 10% of the secondary particles of up to 0.8 ?m, a diameter of 90% of the secondary particles of up to 5 ?m, with a longitudinal size of the primary particles of from 150 to 900 nm, and a thickness of from 15 to 150 nm.