We report on the effect of annealing temperature on the structural and magnetic properties of the pure ZnO nanoparticles (NPs) synthesized using sol-gel route. X-ray diffraction (XRD) patterns confirm the single-phase, a typical wurtzite structure, in all the samples. Crystallite sizes are found to increase with increasing the annealing temperature. The defects and the presence of oxygen vacancy have been estimated through Photoluminescence (PL) and micro-Raman spectroscopic studies. These samples exhibit room temperature ferromagnetism (RT-FM) and the saturation magnetization (MS) decreases gradually with increasing the annealing temperature. The ferromagnetic ordering in ZnO NPs is intrinsic and may be induced via surface defect like oxygen vacancy.