Porous micro-spheres of ZnO have been synthesized with Zn(NO3)26H2 O and urea as the raw materials and hexamine as a surfactant via a hydrothermal method. Structure, morphology, specific surface area, and pore size distribution of ZnO porous micro-sphere are characterized by X-ray diffraction, scanning electron microscopy, and micromeritics ASAP 2420 apparatus, respectively. Gas sensors are fabricated from the ZnO porous micro-spheres and their gas-sensing properties are measured. The gas sensitivity of sensor samples at different temperatures to acetone gas is examined. Results show that the sensitivity of ZnO porous micro-spheres sensors to 50 ppm acetone gas is 26.8 at 280 ℃, and the response and recovery durations are 4-10 seconds, respectively. Also, the sensor possesses an excelent selectivity for acetone.