Magnetic nanoparticles of manganese doped ZnO have been prepared by co-precipitation method. Photocatalytic activity results show that 9 percent of Mn in Mn-doped ZnO exhibited the highest photodegradation efficiency of reactive blue 198 dye. Additionally, excellent photocatalytic activity was observed when the molar ratio of Mn-doped ZnO/Fe3O4 was 1:1. The photodegradation of reactive blue 198 (RB 198) dye was carried out with different illumination times, initial concentrations of dye, amounts of photocatalyst and pH of medium. The results reveal that the degradation efficiency of reactive blue 198 were 99 percent at the concentration of 30 ppm for 25 min when the amount of catalyst was 5 g/L. The photodegradation of dye increased with increasing catalyst load. Furthermore, the prepared magnetic nanoparticles could also serve as convenient recyclable photocatalysts because of their magnetic properties. As a result, the removal of dye was still 90 percent after three times recycling.