This work is concerned in improving the resistance against thermal oxidation of polymer electrolyte based on 30 % methyl grafted natural rubber (MG30) by adding antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) with different weight percent between 0.5 and 8 wt.%. All samples were prepared by solution cast technique. The prepared samples were exposed under 70°C for 3 days. Fourier transform infrared spectroscopy (FTIR) spectral analysis show that MG30 with 1 wt.% of 6PPD is the most effective composition to reduce the thermal oxidation at elevated temperature. The comparative studies are performed to investigate the conductivity performance of both systems of MG30-lithium triflate salt before and after treated with 6PPD. The analysis found that the untreated sample shows better ionic conductivity at room temperature. The modulus formalism study confirms both systems are ionic conductor.