Abstarct: CuO nanoparticles were prepaired by the precipitation of soloution method and characterizied by the FT-IR, SEM and XRD techniques. Then surface of nanoparticles were modified by Oleic acid. CuO/PMMA nanocomposite was prepared from modified CuO nanoparticles and MMA by the emulsion polymerization. The nanoparticles, modified nanoparticles and obtained nanocomposite were used as a photocatalyst to study the photodegradation of methyl orange. The results show that photocatalytic activity of methyl orang CuO nanoparticles modified by oleic acid and nanocomposite is reduced.