More than 50% of the flame retardants used in the polymer and plastic industries are metal hydroxides. Among them, aluminum trihydroxide (ATH) is the most widely used due to its low toxicity and corrosiveness and its cost-effectiveness. The use of high-volume ATH in polymers reduces the mechanical properties. In this work, clay nanoparticles were added into epoxy/glass fiber/ATH hybrid composites to improve the mechanical and thermal properties of the composites. The effect of nano-clay content (1, 3, and 5 phr) on mechanical properties such as tensile and flexural strength and modulus, and thermal properties was investigated. Thermal properties were evaluated by Differential Scanning Calorimetry (DSC), and Thermogravimetric Analysis (TGA). The result of mechanical tests showed that adding 3 phr of clay nanoparticles increases tensile and flexural strength by 10% and 9.2%, respectively. The flammability of the composites was measured in horizontal mode. The flammability results revealed that introducing 3 phr of nano-clay improves the flammability of the composites by 41%.