Flower-like ZnO 3D microstructures composed of nanorods have been successfully prepared via a facile hydrothermal method using p-nitrobenzoic acid as the structure-directing agent. The structures and morphologies of the final products have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM). The possible mechanism for the synthesis of the flower-like ZnO microstructures has been proposed primarily. The gas sensitivity of the flower-like ZnO microstructures has been studied to a series of organic vapors at different operation temperatures and vapor concentrations. The results show that the flower-like ZnO microstructures composed of nanorods have good gas sensor properties to ethanol.