Novel monoclinic CuO nanobundles, 0.8-1 microm in size, were synthesized at 130 degrees C in the presence of sodium dodecyl benzenesulfonate (SDBS) by a simple hydrothermal method. Each nanobundle was comprised of many nanorods with one ends growing together to form a center and another ends radiating laterally from this center. The length and the diameter of these assembled nanorods are in the range of 200-300 nm and about 20-30 nm, respectively. HRTEM and SAED results indicated that the CuO nanorods grow along the [010] direction. An investigation of the hydrothermal process revealed that the reaction time, temperature and surfactant play important roles in the formation of the resultant CuO nanostructures. Isolated CuO nanorods were obtained when the temperature was increased to 190 degrees C, and CuO microflowers composed of many nanosheets were produced at 130 degrees C when cetyltrimethylammonium bromide (CTAB) was employed instead of SDBS. The possible mechanism for the formation of these CuO nanostructures was discussed simply on the basis of the experimental results.