Uniform peachstone-like CuO 3D architectures consisting of single-crystal nanosheets have been successfully synthesized by using ionic liquid 1-octyl-3-methylimidazolium trifluoroacetate ([Omim]TA) as capping reagents under the ionic liquid-assisted hydrothermal condition. Detailed proofs indicated that the process of crystal growth was dominated by an oriented aggregation and self-assemble growth mechanism. The morphology of CuO evolved from nanoparticles to two-dimensional (2D) nanosheets and three-dimensional (3D) peachstone-like nanostructures. A formation process is proposed to illustrate the growth of peachstone-like CuO crystal. The influence of the ionic liquid cations on the morphology of CuO materials was studied in detail. The cations of the ionic liquids control the morphology of crystals. Additionally, it was also found that the concentration of ionic liquids and the reaction time have direct influences on themorphology of the products. Their optical absorption spectra were also studied. The synthetic strategy could be extended to assemble 3D architectures of other materials. (C) 2009 Elsevier B. V. All rights reserved.