In this research, we maed the sheet of unsaturated polyester (UPE) filled with the particles of aluminum hydroxide (AH)-PMMA composite. The content of AH in the AH-PMMA particle was 40%, The particle was coated by cross-linking PMMA made interpenetrating polymer network (IPN) with UPE. The AH-PMMA particle characterized by SEM, TGA, XRD and DSC. The composite sheet of UPE was measured for tensile strength, impact strength. We learned that the particles of AH-PMMA reinforce for tensile strength and impact strength on the UPE composite.