This book consists of synthesis and characterization of ZnO and Mn-doped ZnO ?nanoparticles for photocatalyst process. The work contains synthesis (co- precipitation), ?Characterization (TEM, SEM, EDX, FTIR, XRD, BET, UV-visible NIR) and ?application of ZnO and Mn-doped ZnO nanoparticles. The application included ?photodegradation of m-, o- and p-cresol that was carried out under UV and visible ?irradiation at room temperature, atmospheric pressure, and different PH by batch ?photoreactors. The intermediates were identified by UPLC and mineralization was ?confirmed by TOC measurement. As a result, 1%wt Mn-doped ZnO with lower ?particles size, higher surface area, high cresols adsorption, lower agglomerate, ?appropriate band gap and higher photodegradation ability than other % of Mn-doped ?ZnO is the best photocatalyst that may enhance the photocatalyst activity of ZnO ?under visible light.?