Abstract: The effects of the heat absorbed during decomposition of some flame retardant fillers on the heat release rate measurement were studied using the cone calorimeter. It was found that the endothermic effect of the filler decomposition will affect the heat release rate measured by the cone calorimeter especially at higher levels of loading. Without appropriate corrections, the heat release rate measured by the cone calorimeter would be overestimated for high endothermic materials. Some typical materials were studied quantitatively, including alumina trihydrate and magnesium hydroxide. For alumina trihydrate and magnesium hydroxide, at the loading level of 80% in polymers, high overestimation could be produced, depending on the individual polymer used. A simple correction procedure is proposed to correct the heat release rate measurement using the cone calorimeter by modifying the effective heat of combustion using pyrolysis enthalpy. The corrected results and the comparison with the experimental values measured by the cone calorimeter are presented in the paper.