In the present work, NiO nanoparticles are synthesized using chemical co-precipitation method which is an easy and cost-effective approach. Aqueous solutions of the precursors containing Ni(NO3)2.6H2O and NaOH pellets are calcined at 300 °C for 2 h. The structural, morphological, compositional, optical study, and thermal analysis of the synthesized powdered product was investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), UV–vis absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR) and thermo-gravimetric analysis. FTIR study shows the presence of functional groups in the synthesized NiO nanoparticles. The powder XRD pattern along with Rietveld refinement data reveals that the prepared sample is NiO with face-centered cubic (fcc) structure (Fm-3 m space group) having average crystallite size ranging from 4 to 6 nm. The optical band gap of the synthesized NiO nanoparticles (estimated from Tauc’s plot) is found to be ~ 3.20 eV which is less than the bulk NiO clearly showing red shift. This could be attributed due to the presence of chemical defects or vacancies resulting in the formation of some trap states.