Abstarct: Surface of CuO nanoparticles was modified bytwo fatty acids (Stearic and Palmitic acids) in chloroform at room temperature. The modified CuO nanoparticles were characterized by Fourier transform infrared spectrometry (FT–IR), scanning electron microscopy (SEM), X-ray diffraction (XRD) and elemental analysis. The degree of lipophilic surface of nanoparticles and the rate of their dispersion in organic medium were studied. Finally, the modified and unmodified CuO nanoparticles were used as a catalyst to study the photo degradation of dyes such as methyl orange. When the absorbance of the solutions were measured by UV–Vis spectroscopic method, the results showed that photocatalytic activity on methyl orange CuO nanoparticles surface modified by fatty acid is reduced. The surface of CuO nanoparticles modified by fatty acids show good dispersion of nanoparticles, form stable suspension in CCl4 solution, have appropriate compatibility with organic medium and therefore reduce accumulation and agglomeration.