Nanocrystalline cobalt hydroxide and cobalt oxide compounds have been prepared using ultrasound-assisted method and some of their optical and structural properties have been characterized by thermogravimetric analyses(TGA), X-ray diffraction (XRD), scanning electron microscope (SEM), and UV-Visible spectroscopy. The results of thermogravimetric analyses show a sharp change in TGA peak, and the oneset of the peak (at 250°C) is transition temperature from hydroxide to oxide of the product. The optical absorption studies show that there are two direct band gaps in Co3O4 nanostructures. The XRD patterns of the prepared samples shows formation of pure cobalt oxide in cubic structure and cobalt hydroxide compound in hexagonal structure with average grain sizes are below 30 nm. Surface morphology of the prepared structures shows that the nanoparticles from nearly globular aggregates of microdimensions.