In this study, we successfully synthesized Cr 3+ doped ZnO nanoparticles by using a sol-gel method, and elucidated how Cr3+ dopant is critical for enhancing photocatalytic activity. The crystalline structure characterized by using X-ray diffractometry showed that the hexagonal wurtzite lattice constant (c) was decreased when the Cr3+ content increased. The nature of the point defect analyzed by electron spin resonance (ESR),and photoluminescence (PL) emission revealed the role of the Cr3+ dopant. When increasing the Cr3+ content in ZnO, the intensity of PL emission was reduced, indicating a reduction of the radiative recombination rate due to a formation of the heterojunction between the Cr 3+ ions and ZnO. The Cr3+ doped ZnO nanostructures showed that the typical ESR signal with g-factor ~1.96 was completely passivated, indicating the diffusion of electrons near the conduction band into the dopant ions. The doped Cr 3+ ion acts as an electron trap in the ZnO crystal described as. The mechanism for enhancing the photocatalytic activity of 3 2Cr e Cr heterogeneous ZnO:Cr3+ was proposed in respect of point defect evolution through the manner of Cr3+ doping. As a result, the photocatalytic efficiency investigated by measuring methylene blue degradation under 210 min of direct sunlight irradiation reached 93.5% for 1 at.% Cr3+  doped ZnO,  which was significantly improved as compared to 59.8% of the pure ZnO.