The objective of this work was to evaluate the effect of the addition of cagaita leaf ethanolic extract (EFC) on soybean methyl biodiesel samples and to compare the effect of oxidative degradation during the storage period of 120 days. The quercetin, a natural antioxidante, was added to biodiesel samples for comparison purposes. All samples, after addition, were stored at room temperature and protected from light. Oxidative stability was monitored by the induction period through the Rancimat (R) method during the storage period of 120 days. It was also evaluated the antioxidant activity of the extract, by the DPPH (2,2-diphenyl-1-picrylhydrazyl) method and the phenolic compounds content, by the method of Folin Cicateou. The phytochemical prospection of the extract indicated the presence of flavonoids, tannins and steroids/triterpenoids, and FT-IR spectrum analysis showed bands of phenolic compounds. The 50 ppm solution of the extract showed the percentage of antioxidant activity of 96.52% and the content of phenolic compounds was 876.46 mg gallic acid equivalent (EAG)/g EFC. The EFC showed to be efficient in retarding the oxidation of biodiesel, since the induction period increased from 4.53 h to 6.04 h. After 120 days of storage, the samples with 800 ppm or more had the induction period higher than that recommended by EN 14214 and samples added with the extract were more effective in retarding the oxidation of biodiesel than quercetin, which is a known antioxidant and widely used, thus confirming the potential use of EFC to retard biodiesel oxidation. (c) 2020 Elsevier Ltd. All rights reserved.