Cobalt oxide nanostructures have been prepared using ultrasound waves by adding different concentrations of oleic acid as a capping agent. Cobalt oxide nanostructures were characterized using different techniques such as scanning electron microscopy, vibrating sample magnetometer (VSM), thermogravimetric analysis (TGA), differential thermal analysis (DTA), X-ray diffraction (XRD), UV-Vis, and FT-IR spectroscopy. The XRD patterns shows formation of pure cobalt oxide compounds in cubic structure. UV-Vis spectroscopy showed that by increasing the concentration of capping agent, the wavelength of absorption edge of the samples shifted toward blue region of the spectrum. The obtained results from VSM, indicated that the variation of magnetic susceptibility increased or decreased with concentration of oleic acid.