Flower-like ZnO microstructures have been fabricated by a facile microwave hydrothermal method (HM) with the aid of benzoic acid. The obtained products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The morphology gradually evolved from flower bud shape to fireworks display with increase of alkalinity in the presence of benzoic acid. The formation mechanism of the flower-like ZnO microstructures was investigated briefly. Furthermore, the gas response of the flower-like ZnO microstructures has been studied to a series of organic vapors. It was found that the gas sensing properties were influenced by the size of the ZnO microstructures. The facile preparation method and the improved gas-sensing properties derived from the flower-like ZnO microstructures demonstrated their potential applications in gas sensor.