Nanocrystalline copper oxide (CuO) thin films have been synthesized by a sol–gel method using cupric acetate Cu (CH3COO) as a precursor. The as prepared powder was sintered at various temperatures in the range of (300–700 °C) and has been deposited onto a glass substrates using spin coating technique. The structural, compositional, morphological, electrical optical and gas sensing properties of CuO thin films have been studied by X-ray diffraction, Scanning Electron Microscopy (SEM), Four Probe Resistivity measurement and UV–visible spectrophotometer. The variation in annealing temperature affected the film morphology and optoelectronic properties. X-ray diffraction patterns of CuO films show that all the films are nanocrystallized in the monoclinic structure and present a random orientation. The crystallite size increases with increasing annealing temperature (40–45 nm).The room temperature dc electrical conductivity was increased from 10−6 to 10−5 (Ω cm)−1, after annealing due to the removal of H2O vapor which may resist conduction between CuO grain. The thermopower measurement shows that CuO films were found of n-type, apparently suggesting the existence of oxygen vacancies in the structure. The electron carrier concentration (n) and mobility (μ) of CuO films annealed at 400–700 °C were estimated to be of the order of 4.6–7.2 × 1019 cm−3 and 3.7–5.4 × 10−5 cm2 V−1 s−1 respectively. It is observed that CuO thin film annealing at 700 °C after deposition provide a smooth and flat texture suited for optoelectronic applications. The optical band gap energy decreases (1.64–1.46 eV) with increasing annealing temperature. It was observed that the crystallite size increases with increasing annealing temperature. These modifications influence the morphology, electrical and optical properties.