Face-centered-cubic (fcc) and hexagonal-close-packed (hcp) phases of cobalt monoxide (CoO) nanostructures are prepared using thermolysis route at the same reaction temperature 296 °C with synthetic approach conditions. These nanostructures show mixture of nearly spherical and nanoflake morphologies. The structural phases of these nanostructures transform to spinel-Co 3O4 by application of heat or Raman excitation laser beam power. The absorbance spectra of fcc and hcp-CoO and Co3O4 nanostructures yield significantly higher values of band gap which can be explained by electron confinement. Such results provide new opportunities for optimizing and enhancing the properties and performance of cobalt oxide nanomaterial. Copyright © 2014 American Scientific Publishers All rights reserved.