The nanorods bundles (urchinlike) ZnO microspheres, consisting of closely packed nanorods with wideness of about 50 nm, have been successfully synthesized by a hydrothermal route at the temperature of 140°C. The individual hierarchical ZnO microsphere ranged from 3.5 to 4.5 μm in diameter. Most importantly, ZnO hierarchical microsphere sensor exhibits excellent selectivity and fast response to acetone. Response and recovery times were 1 s and 3 s when the sensor was exposed to 100 ppm acetone at an operating temperature of 320°C. Thus, hierarchical structures play a significant role in the field of gas sensing.