Since ancient times, medicinal plants were used for the treatment of human diseases. The wide range of bioactive compounds contained in plants stipulates the development of novel phytopharmaceuticals. Traditional phytotherapy is also still applied as part of complex treatment. Antioxidants are one of the largest groups of bioactive compounds of plant origin. Thus, evaluation of the antioxidant capacity of medicinal plant extracts used in phytotherapy is of practical interest. Water extracts from 11 plants obtained by sonication for 30 min have been studied by cyclic voltammetry at bare glassy carbon electrode and electrode modified with mixture of 1 mg mL^–1 CeO₂ and SnO₂ nanoparticles dispersed in 0.10 mM cetylpyridinium bromide. Electrode surface modification provides improvement of the extract response in the shape of oxidation steps and their currents, which is caused by increase in the effective surface area of the modified electrode as voltammetric data confirm. For all extracts under study, the first oxidation step occurs in the range of 310-400 mV and the second one – in the range of 520-900 mV that agrees well with the classification of antioxidants by their reducing power. As far as only components with relatively high concentration give response on the voltammograms, a two-step chronoamperometric approach has been developed for estimation of the antioxidant capacity of medicinal plant extracts. Components with low concentration also give impact in the analytical response in this case. Two steps at 400 and 900 mV for 75 s each one have been applied and antioxidant capacity has been expressed as current recalculated per 100 mL of the extract. The comparison of the data obtained with the standard antioxidant parameters (total phenolics contents and antioxidant capacity toward 2,2-diphenyl-1-picrylhydrazyl) has shown a strong and very strong correlation level.