The poly(diphenylamine) derivative (PDPA) as amine antioxidant was synthesized via diphenylamine, styrene, and formaldehyde. The steps of the synthesis were discussed in detail, and the structure of PDPA was fully characterized with multiple analysis techniques. Furthermore, the anti-oxidative properties of PDPA were evaluated through oxidation-corrosion tests and differential scanning calorimetry. It was found that PDPA significantly improved the oxidation stability of base oils, especially pentaerythritol ester when it was added at concentrations of only 0.5 wt % to 0.8 wt %. Thermogravimetric analysis shows that anti-oxidation behavior of PDPA is better than traditional additive p, p-dioctylphenylamine. The synthesized PDPA antioxidant is a potential alternate additive in high-temperature conditions.