ZnO nanoparticles were synthesized by the spray-pyrolysis method. There were exposed to oxygen plasma treatment to increase OH radicals on their surfaces. ZnO nanoparticles were spherical grains with a narrow size range of about 7-10 nm, and these small particles agglomerated together to form larger clusters. Synthesized ZnO nanoparticles and a p-25 mixed MB solution exposed to UV illustrate that p-25 has a better photo-degradation response than synthesized ZnO nanoparticles using UV light. Therefore, we did oxygen plasma treatment to increase OH radicals on the surface of synthesized ZnO nanoparticles. As a result, the photocatalytic activity of oxygen plasma treated ZnO nanoparticles was enhanced by about 60% based on the absorption peak of the decomposed MB solution. Also, we have analyzed the surface of oxygen plasma treated ZnO nanoparticles by XPS. Results showed that OH radicals and molecular water on the ZnO surface are enhanced by oxygen plasma treatment. Also, treatment reduced oxygen and zinc vacancies on the treated ZnO surface. (C) 2015 Elsevier B.V. All rights reserved.