Abstarct: Surface of NiO nanoparticles was modifiedby the two fatty acids (palmitic acid and stearic acid) in chloroform at room temperature. The modified NiO particles were characterized by Fourier transforminfrared spectrometry (FT- IR), X- ray diffraction (XRD), scanning electron microscopy (SEM) and elemental analysis. Then degree of lipophilic properties of nanoparticles and also the rate of their dispersion in organic medium such as acetone and liquid paraffin were measured. Finally, the modified and unmodified nanoparticles were used as the catalyst in photo degradation of dyes such as methyl orange. UV- Vis spectroscopic method was used to determine the absorbance of the solution. The results showed that photocatalyticactivity on methyl orange with NiO nanoparticles surface modified by fatty acids is reduced. The features of NiO nanoparticles surface modified by fatty acids prosses include good dispersion of nanoparticles, stable suspension in acetone solvent, appropriate compatibility with organic medium, reduce the accumulation and agglomeration.