Copper(II) oxide nanoparticles with an average size of 52 +/- 5 nm are synthesized by wet combustion of the extract of coronate saw-wort (Serratula coronataL.) growing on the territory of Central Kazakhstan. The complex study of the structure and composition of the synthesized nanoparticles by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray phase analysis shows that the nanoparticles contain no additional impurities, have the monoclinic structure, and possess a high degree of crystallinity; the average size of crystallites is 28 +/- 4 nm. Catalytic activity is tested in methylene blue dye degradation under exposure to visible light (500 W, 7500 lm). The degradation efficiency is studied as a function of catalyst mass and initial dye concentration. It is shown that even at a catalyst loading of 10 mg more than 54% of the dye degrades in the reaction mixture. The study of catalyst stability demonstrates that the efficiency of degradation decreases by 6.1 and 33.3% after the second and fifth test cycle, respectively.