Abstract. In this study, CuO nanostructures were synthesized through facile precipitation route followed by a typical annealing treatment at higher temperature and prolonged time. The SEM analysis revealed that the CuO exhibited various morphological transitions, namely sphere-like, sponge-like, and fiber-like nanostructure depending on the annealing temperature and time. According to the FTIR result, the high-intensity sharp peaks occurred between 500 and 850 cm^-1 represent the formation of pure vibrational stretching Cu-O bonds. CuO that underwent annealing treatment at higher temperature and prolonged heating time resulted in finer nanostructure formation, crystallinity improvement, and optical band gap narrowing. These features allow CuO to potentially be used as photocatalyst to degrade the aqueous organic dye pollutants for wastewater remediation applications.

Key words: CuO nanostructure, morphological, optical properties.