Abstract: Microencapsulated Mg(OH)₂ (M-Mg(OH)₂) flame retardant with micro-sized Mg(OH)₂ particles as the core and cross-linking polyurea as the shell, was synthesized by in-situ polymerization. The obtained M-Mg(OH)₂ was incorporated into ethylene-vinyl acetate copolymer (EVA) and its flame retardant effect on EVA was studied. FTIR, SEM, thermal analysis and acid titration were used to investigate the properties of M-Mg(OH)₂. Limiting oxygen index (LOI) and vertical burning test (UL-94 VBT) were employed to study the flame retardancy of M-Mg(OH)₂/EVA composite and the effect of acid erosion on the flame retardancy of M-Mg(OH)₂/EVA composite. It shows that the surface of Mg(OH)₂ particles can be successfully encapsulated with cross-linking polyurea by in-situ polymerization and microencapsulated Mg(OH)₂ can be obtained. Compared with the pure Mg(OH)₂, the size of M-Mg(OH)₂ is larger, its thermal stability increases and the water solubility of M-Mg(OH)₂ decreases remarkably. The dispersion of M-Mg(OH)₂ in EVA matrix is more even than that of the pure Mg(OH)₂. The LOI of the M-Mg(OH)₂/EVA composite is always slightly larger than that of its Mg(OH)₂/EVA counterpart at identical loadings of flame retardant (FR). When the mass ratio of FR/EVA is less than 135:100, the VBT ratings of both composites are V-2. When the mass ratio of FR/EVA is between 135:100-150:100, the rating of the former is V-0, but the latter can only reach V-2 rating. When the mass ratio of FR/EVA surpasses 150:100, both composites can reach V-0 rating. In contrast with Mg(OH)₂/EVA composite, the acid resistance of the M-Mg(OH)₂/EVA composite increases substantially and it can be used in acid environment.