In this paper, we reported that the annealing at temperatures of 400 and 500 oC in the air for 2 hours led to the formation of rod-like structures of cupric oxide thin films prepared by the chemical bath deposition technique. The structure and the optical properties of the prepared thin films were studied to investigate the role of annealing on the films. The morphology of the as-deposited CuO films is almost structureless. However, the films are converted to rod-like shapes nano-structures after annealing, as confirmed by scanning electron microscopy. The x-ray analysis showed that the thin films of copper oxide nano-structures have a monoclinic crystallinity preferred in the (110), (002), and (111) directions, and the crystallinity increases after annealing. Furthermore, the bandgap values after annealing are reduced from 2.1 to 1.61 and 1.63 eV as determined by optical analysis utilizing UV–VIS spectroscopy.