Zinc oxide nanostructures were prepared by microwave-irradiation technique in the presence of polyvinyl pyrrolidone as a shape modifier. The synthesized nanostructures were analyzed using scanning and transmission electron microscopy, selected-area electron diffraction, X-ray diffraction, and Fourier transform infrared spectroscopy (FT-IR). Characterization techniques revealed the formation of crystalline ZnO with a star-like shaped morphology, having the crystal structure of wurtzite. The microwave irradiated samples were further investigated to exploit their electrical and sensing properties. The good sensitivity and relatively short response and recovery times of chemoresistive sensors based on star-like ZnO thick films in the monitoring of low concentration of ammonia gas in air were related to the peculiar nanostructure of the sensing layer.