Rotary kilns are widely used in industry for heat treatment of granular materials due to their large capacity. The continuous thermal processing of a large number of materials determines that the rotary kiln is energy intensive. It is important to well understand the thermal mixing and heat transfer processes in rotary kilns. But the interaction of drum size, operating condition and material properties complicates the temperature distribution in solid bed. The dynamic changes of solid temperature profiles are still unclear. To study the thermal mixing of the solid bed in a rotary kiln, a mathematical model for thermal mixing in a rotary kiln is presented by coupling the DEM (discrete element method) with the heat transfer mechanism. The model is validated with experiments in a batch operated indirectly heated rotary kiln with an inner diameter of 0.6 m. The drum is operated under various drum filling degrees (10 − 20%), rotational speeds (1 − 6 rpm) and different ratios of hot and cold particles. The results show that the thermal mixing time and the bed revolution time increase with increasing drum filling degree and decreasing drum rotating speed. Drum filling degree has little influence on the number of bed revolutions. Besides, different ratios of hot and cold particles under a constant drum filling degree show similar heat transfer characteristics. In general, DEM simulations can predict the thermal mixing behavior of the solid bed in a rotary kiln well.