Abstract: Thermal decomposition experiments of limestone with five different particle sizes were conducted through thermo gravimetric analyzer to study its calcined characteristics with large particle size.The limestone particle sizes were distributed in the range of 0.5~10 mm，and the temperature was controlled at 900~1 050 ℃ during the experiments.Rresults revealed that the smaller the particle size，the higher the temperature and the faster the decomposition.And the particle size and calcining temperature made an obvious influence on the thermal decomposition mechanism.When the particle size was small and the temperature was high，the reaction was in accordance with the randomly nucleating and nucleus growth model，but in contrast，reaction was in accordance with the phase interface mechanism.When the particle size was  at 0.5~1 mm， reaction activation energy was very small，and with the increase of particle size，reaction activation energy increased，while in the range of 1~5 mm，activation energy fluctuated，with the continuous increase of particle size to 5~10 mm，the activation energy doubled.

Key words: limestone, thermo gravimetric analysis, particle size, thermal decomposition kinetics