Abstract: A series of Mg(OH)₂ with different degradation states (t-MH) were obtained by pyrolyzing Mg(OH)₂ (Magnesium hydroxide, MH) at various temperatures for different time. MH/PS and t-MH/PS composites were prepared by melt-compounding MH and t-MH as halogen-free flame retardants and PS as matrix. The influence of thermal treatment of MH on combustion behavior of t-MH/PS composites was investigated in detail. Based on the experimental results, a model of the flame retardant mechanism of MH/PS composites was proposed. It is indicated that MH gradually decomposes and changes into MgO with the rise of thermal treatment temperature and the elongation of thermal treatment time. Under identical conditions, the flame retardancy of t-MH/PS composite decreases noticeably than that of its MH/PS counterpart, but it is still much better than that of pure PS. In addition, the MgO produced by decomposition of MH can promote charring of PS. It is believed that the flame retardant mechanism of MH/PS composites derives from the synergistic effect of the following factors: (1) the cooling effect from endothermal degradation of MH; (2) the gas phase diluting effect of water vapor from degradation of MH; (3) the solid phase obstructing and shielding effect of MgO from degradation of MH; (4) the fire-proof effect from charring of PS induced by MgO.