The non-isothermal pyrolysis kinetics of micron Mg(OH)2 is studied with the thermogravimetric analyzer under nitrogen atmosphere. It mainly focuses on clarifying the effects of particle size on the pyrolysis process. The measured thermogravimetric curves show a stepwise characteristic. At the main decomposition stage, the pyrolysis of Mg(OH)2 is rapid with a sudden mass loss. At the subsequent stage where conversion rate is larger than 0.8, the rate of weight loss decreases significantly because the diffusion of product water vapor is hindered by the product of MgO film. As for the main decomposition stage with conversion rate of 0~0.8, the Starink method and combined kinetic analysis are applied to fit and analyze its kinetic model. It is found that particle size has no distinct effect on the pyrolysis kinetics of micron Mg(OH)2. The model fitting results show that the pyrolysis reaction activation energy of micron Mg(OH)2 is 129.4 kJ/mol, the pre exponential factor is 1.820×1010 min?1, the pyrolysis reaction process follows the mechanism of random nucleation and growth of nuclei (A1.5). Compared to the experimental results, the thermogravimetric curves reconstructed based on these fitting parameter have the deviation less than 5%, which indicates that the kinetic model is reasonable. Meanwhile, it can be concluded that the pyrolysis process is controlled by decomposition reaction in the investigated range with the conversion rate from 0 to 0.8. The resistance effect of particle heat conduction and product water vapor diffusion is weak relatively. The main process of Mg(OH)2 weight loss is approximate to a homogeneous reaction. Considering the current investigation mainly focuses on the main decomposition stage of Mg(OH)2 with the conversion rate from 0 to 0.8, the kinetics of second stage of weight loss process remains to be studied further.