The biodiesel's importance in the renewable energy scenario is attributed to the lower environmental impact due to sustainability and economic viability. Biodiesel from production and storage is in contact with different metallic materials that catalyze the oxidation reaction and accelerate oxidative degradation. In order to inhibit the oxidative degradation of this biofuel, plant extracts such as rosemary, oregano, basil, sage, and pepper can be added which have phenolic compounds that act as antioxidants. The present research aims to evaluate the influence of iron oxide and iron(III) chloride on the oxidative stability of biodiesel and to identify deviations in reaction activation energy in the presence and absence of natural antioxidant. In the period evaluated, the biodiesel tests with addition of ferric chloride had the lowest oxidative stability; consequently, the oxidative reaction had the highest rate constant. The rate constant of the oxidation reactions increased with the increase in the evaluated period as well as the temperature employed. The samples with ferric chloride showed second-order polynomial behavior, indicating that the activation energy (E-a) is dependent on the reaction temperature.