Elevated temperature brittleness (ETB) of ferritic ductile cast iron has been referred to as reduced ductility within an elevated temperature range and has been related to grain boundary brittleness. The phenomenon of ETB has not been yet clearly understood. In this study, the mechanism of ETB was studied in terms of strain rate and chemical composition. Yield strength was related to solid solution hardening of silicon. Serration, which was observed at temperatures between 425K and 625K, was suppressed by increasing strain rate and adding molybdenum. The reduced tensile ductility was the direct result of intergranular embrittlement and the temperature, at which ductility was minimum, was strain-dependent and was about 700K. Heats containing low phosphorus were found to be immune to ETB, indicating silicon and molybdenum had little effects on it.