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This study aimed to achieve a formulation for an additive to produce oxo-biodegradable films that accelerates oxidative degradation of the films after preservation of properties over a span of desired service life. Thermal oxidation behavior of high-density polyethylene (HDPE) films (approximately 250 μm thick) containing various weight ratios of a commercially used phenolic antioxidant (Irganox 1010) to ferric stearate as pro-oxidant has been studied in both melt and solid states. Thermo-oxidative stability in melt state was studied using differential scanning calorimetry. The rate of thermal oxidation in solid state was investigated via oven aging experiments at 90 °C followed by measuring changes in tensile properties, gel content, carbonyl index and density. Comparing thermo-oxidative stability of the HDPE samples containing a combination of Irganox 1010 and ferric stearate with the samples containing Irganox 1010 alone confirmed that ferric stearate reduces the stabilization efficiency of the phenolic antioxidant in the polymer either in melt or in solid state. It was also shown that the efficiency of the phenolic antioxidant in thermo-oxidative stabilization of the polymer in both melt and solid states could be changed by altering weight ratio of Irganox 1010/ferric stearate. On the basis of the obtained results, it was concluded that weight ratio of 0.1/0.1 wt% of the antioxidant to the pro-oxidant is suitable for attaining desired stability during melt processing as well as retaining properties during a reasonable service life when is used as a film and a favorable rate of thermal oxidation after the service life.