The flowerlike ZnO nanorod bundles, consisting of closely packed nanorods with diameter of 90 nm, have been successfully synthesized by a poly(ethylene glycol) (PEG)-assisted hydrothermal route at low temperature (80 °C). The results characterized from FESEM, TEM, and SAED demonstrate that the nanorod structures are single crystals and formed from self-assembled nanoparticles. Further investigation of the formation mechanism reveals that the PEG-assisted hydrothermal process is vital to the formation of the complex nanostructures. The sensors based on the ZnO nanostructures exhibit excellent ethanol-sensing properties at reduced working temperature (250 °C), which could still respond to 1 ppm ethanol. The sensitivity of the sensor to 100 ppm ethanol is about 154.3 with the response time of 12 s. The enhancement in sensing properties of the present ZnO sensor may be attributed to the peculiar ZnO nanostructures.