The uncapped and capped CuO nanoparticles were synthesized by sol–gel process. Ethylene diamine tetra acetic acid (EDTA) used as a capping agent to control the size and morphology of the CuO nanoparticles. Control the nanostructure and optical properties of CuO nanoparticles are deliberate by using capping agent. The obtained samples were characterized by means of UV–Visible spectroscopy (UV–Vis), X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM) along with energy dispersive X-ray spectroscopy (EDS). UV–Visible technique used to measure the band gap of the synthesized samples. The XRD spectra of the synthesized samples confirmed the CuO nanoparticles with monoclinic structure. The occurrences of capping agent have been analyzed by Fourier transform-infrared spectroscopy (FT-IR). The FT-IR study revealed that the transmittance peaks at 501cm −1 and 592cm −1 were CuO stretching mode, and 677cm −1 was CuO monoclinic phase of the uncapped CuO nanoparticles. Further, SEM study find out the morphology of the synthesized samples, where the surface morphology changes occurred with an addition of capping agent. Moreover, the capping mechanism has been discussed based on the chemical reaction between the CuO nanoparticles and the functional groups of the capping agent.