The influence of refractory fine-grained fillers of different physical nature on the heat resistance of epoxy composites in the temperature range was set. Composites based on epoxy diane resin ED-20 grade that was cross-linked by low molecular polyethylene polyamine hardener were investigated. Tris(2-chloroethyl) phosphate was used as plasticizer. The study was held with the input of refractory fine-grained fillers such as sovelite powder, calcium carbonate, sodium dichloroisocyanurate. Composite materials thermal stability, which characterizes the ultimate weight loss temperature was set by thermogravimetric and differential thermal analysis. It is proved that composite materials filled with particles of calcium carbonate with the highest ultimate weight loss temperature of , are characterized by improved thermal resistance. It was found that composites with particles of calcium carbonate are characterized by the lowest relative mass loss . Based on indicators of experimental investigations the optimum concentration of calcium carbonate in the composite material of was reasoned. Such filler content inhibits thermal decomposition and is a fire retardant, that allows additionally improve the physical and mechanical properties of materials. Also the activation energy of the developed composites was investigated in the paper. On the basis of studies, the concentrations of investigated fillers were established at which thermal decomposition of epoxy composites and the highest activation energy are observed. The reliability of the obtained data was proved by the correlation of experimental results with previous studies of thermal properties and the use of modern methods of research – thermogravimetric (TGA) and differential thermal (DTA) analysis, which is additionally confirmed the intensity of thermal field impact on the degradation processes of developed materials.