Internal mixers have been widely used as rubber mixing equipments for more than a century. The optimization for rubber mixing in internal mixers should be required to improve the compound properties and productivity. Then the mixing process requires high energy input, and at the same time such mixing causes an increase in rubber temperature due to the heat generated by shearing force. Therefore heat transfer characteristics in internal mixers are important for the prevention of material deterioration. The paper describes various studies on the heat transfer characteristics between internal mixers and rubber compounds. Comparison between tangential and intermeshing rotor systems is also reported. The advantage of intermeshing system is higher mixing and cooling performance in terms of heat transfer characteristics.