The viscosity of natural rubber fluctuates more than that of other rubbers during processing. In its raw material state, natural rubber is very hard, and it is difficult to add fillers and other materials without kneading, and in the subsequent kneading and molding processes, it is subjected to mechanical shear, oxidation, and other processing history, resulting in a gradual and unintentional decrease in viscosity. In addition, there is a possibility of three-dimensional bonding between rubbers due to polymer radicals, etc., especially under conditions of low oxygen content. In this paper, we focus on “processing stability of natural rubber”and discuss how processing aids can contribute to the stability of natural rubber.