Microwave irradiation was used to obtain a variety of CuO crystal morphologies, including leaf-like, dandelion-like, and hollow structures. The morphology of the CuO crystals was controlled by varying the alkali source (NaOH, hexamethylenetetramine, ammonia, or urea) and heating at 95 ℃ for 1 hr. The X-ray diffraction patterns of as-prepared CuO crystals were consistent with high quality crystals with a monoclinic crystal structure. Field emission scanning electron microscopy (FE-SEM) and tunneling electron microscopy (TEM) images of CuO crystals revealed that the leaf-like CuO crystals had an average length of 950 nm and width of 450 nm, the small leaf-like CuO crystals had an average length of 450 nm and width of 200 nm, the dandelion-like CuO structures had an average diameter of 2 m, and the hollow CuO structures had an average diameter 2 m. Possible mechanisms for structure formation during the shape-selective CuO synthesis were proposed based on these results.