Here we report a simple hydrothermal route for the preparation of CuO nano-flower which has been proved to exhibit good performances in gas-sensing. We concentrate on the microstructure and morphology of CuO by adjusting the reaction time length. Four different morphologies that formed a flower-like shape with a diameter of 1–3 µm were obtained. The CuO nanostructures were characterized in detail by means of X-ray diffraction and scanning electron microscopy and were apparently seen that they were composed of abundant aggregative nanobelts. The prepared CuO properties in gas-sensing were also measured in our experiments. In addition, a proper growth mechanism was proposed based on the results. Moreover, the CuO nano-flowers may resolve the difficulties in low-dimensional gas-sensing applications after more research having been conducted.