The ZnO nanoflower morphology was successfully fabricated by the hydrothermal method. The samples were characterized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), and diffuse reflectance spectrophotometry (DRS). Photocatalytic activity of the nanostructures against P-nitrophenol was studied under ultra-violet (UV) and visible light irradiation. The more porous nanoflower powder exhibits higher visible light absorbance, lower optical band gap (~ 3.07 eV), substantially more surface area, and, consequently, stronger photocatalytic activity. The kinetic-constant of the photocatalytic degradation of ZnO nanoflowers follows from the pseudo-first-order kinetic model and was found to be 0.0198 (under UV) and 0.0038 (under visible light), which were ∼ 1.6 times stronger than that of commercial ZnO nanopowder.