ZnO spheres composed of porous flakes were prepared by template-free, economical hydrothermal method combined with heat treatment. The high-density pores with the size in tens of nanometers were found distributed through each piece of the flake. The sensor based on porous ZnO spheres showed improved ethanol response compared to that based on commercial ZnO powders. ZnO spheres sensor showed obvious response to 2 ppm ethanol at the operation temperature of 280 A degrees C, with the corresponding response value of 2.8 and the response and recovery times of 10 and 15 s, respectively. The response of the sensor increased linearly with the concentration of ethanol in the range of 2-500 ppm. The good sensing performance of the ZnO sphere sensor to ethanol gas indicated that the ZnO sphere could be a promising candidate as the building block for highly sensitive and reliable gas sensor.