The aim of this study was to perform phytochemical screening, to quantify the phenolic compound and to evaluate the antioxidant activity of the ethanolic, ethanolic-water and aqueous extracts of Mitragyna ciliata and Terminalia catappa, medicinal plants from Gabon. Phytochemical tests reveal that both plants contain alkaloids, flavonoids, terpenoids, saponosids, tannins and reducing compound. The quantitative analysis shows that the polar extracts of Terminalia catappa have high contents in phenolic compounds compared to the polar extracts of Mitragyna ciliata which have low contents. The polar extracts of Terminalia catappa, with respective inhibition concentrations of 0.27 ± 0.114; 0.102 ± 0.061 and 0.97 ± 0.068 mg.mL-1, are on average given a more significant antioxidant power compared to that of the control (Vitamin C). The polar extracts of Mitragyna ciliata have weak antioxidant activistes with respective inhibition concentrations of 1.026 ± 0.061; 0.804 ± 0.071 and 1.16 ± 0.055 mg.mL-1.