A mild template-free mixed solution medium with the assistant of microwave method was successfully established to synthesize well-aligned CuO nanostructures. By varying process parameters such as the volume ratio of the mixed solution, different kinds of architectural structures can be controllably synthesized in large quantities. On the basis of the results, it is found that the polarity of the solution plays the key role in controlling the growth of the CuO crystal. The as-prepared CuO products were characterized using diverse techniques including X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The gas sensor property investigation shows that the sensitivities to ethanol of the as-prepared CuO nanostructures are higher than those of normal CuO nanoparticles. Published by Elsevier B. V.