Hexagonal nano-magnesium hydroxide with thickness of 25—30 nm and diameter of 0.3—0.4 μm was synthesized via acid immersion and ammonia precipitation from dolomites. Its quality met the standards of typeⅠ in HG/ T 3607—2000. The thermal decomposition kinetics of Mg(OH)2 was investigated by using thermal gravimetric analysis at different heating rates for deep understanding of physical chemistry in this process. The activation energies of decomposition estimated by the Kissinger and Ozawa methods were 115.47 kJ·mol-1 and 126.04 kJ·mol-1 respectively. Decomposition reaction of magnesium hydroxide was controlled by Avrami-Erofeev function (n=1.5). Pre-exponential was 3.077×1010 s-1. Nano-magnesium oxide with average diameter of 80—100 nm was prepared after calcination of magnesium hydroxide.