Poly(p-methoxyphenol-phenylamine), denoted as P(MOP-PA), was synthesized via the enzymatic oligomerization of p-methoxyphenol and phenylamine monomers in the presence of horseradish. The structure of the as synthesized product was confirmed by Fourier transform infrared spectrometry, time-Of-flight mass spectrometry, and elemental analysis, and the oligomerization process was studied by high-performance liquid chromatography. Moreover, the antioxidation behavior of P(MOP-PA) as an antioxidant in several ester oils was evaluated by rotary oxygen bomb test and pressurized differential scanning calorimetry, and its antioxidant mechanism was discussed. It was found that, as the antioxidant in various base oils, P(MOP-PA) exhibits excellent antioxidation ability at elevated temperatures of 150 and 210 degrees C. In addition, P(MOP-PA) has an antioxidant ability that is better than that of poly(p-methoxyphenol), and it exhibits an antioxidation ability in synthetic ester oil, such as di-iso-octyl sebacate, that is much better than that of several commonly used commercial hindered phenolic antioxidants.