In this paper, three-dimensional flower-like ZnO hierarchical nanostructures were fabricated from the thermal-decomposition of 3D zinc hydroxide carbonate precursor, which was synthesized by a urea hydrothermal method with block copolymer F127 (EO106-PO70-EO106) as the morphology director. XRD, IR, UV-vis, SEM, TEM, TG and N2 adsorption–desorption isotherms have been employed to characterize the products. The influences of synthesis parameters such as reaction time, the type of zinc sources, and species concentration on the morphologies of the products were systematically studied. It was found that the reaction time played a key role in determining the final morphology of porous ZnO. On the basis of experimental results, a possible formation mechanism of the 3D flower-like ZnO hierarchical nanostructures was discussed. More importantly, the gas sensing tests indicated that the sensor made from porous ZnO hierarchical nanostructures exhibited better gas sensing properties to n-butanol compared with the sensor based on the commercial ZnO nanoparticles. The enhancement in gas sensing properties was attributed to their unique 3D hierarchical nanostructures, high surface areas, and greater number of surface active sites.