The effect of several antioxidants on the activation energy of oxidation of natural rubber was studied by DSC. With the exception of 2,6-di-tert-butyl-p-cresol, the addition of amine and dithiocarbamate types of antioxidants increased the activation energy of oxidation. Pro-oxidant effect was indicated for the dithiocarbamate type antioxidants at higher concentration. Activation energy appeared to provide a reasonable estimation of the effectiveness of antioxidant.