This paper describes the efficiency of photocatalytic degradation of textile dye, reactive blue 4, using ZnO in the form of slurry. The results showed that the dye molecules can be completely degraded to CO2 SO42-, NH4+, NO3- and H2O under solar irradiation. The effects of various parameters viz., ZnO loading, initial concentration of dye, persulphate ion, pH and sodium carbonate on the photocatalytic degradation were investigated. The degradation of dye was found to be effective in alkaline solution. This work envisages the great potential that sunlight mediated photocatalyst has in the removal of textile dyes from wastewater.