The increasing demand for hazardous gas detections has triggered the enormous efforts for the development ofmesoporousmaterials for gas sensors. Especially, a universal method to synthesize such structure for semiconductor oxides is highly desirable. In this work, ZnO micorspheres assembled by mesoporous nanosheets are prepared via a one-step hydrothermal method successfully. It is found that the morphology of ZnO and the thickness of ZnO mesoporous nanosheets, which affect the gas sensing response to ethanol, can be regulated by adjusting the amount of PVP. In addition, this method is also suitable for the synthesis of NiO, CuO and Co₃O₄mesoporous nanosheets and the repeatability is excellent. Gas sensing investigation indicates that the gas sensors based on mesoporous ZnO nanospheres synthesized with 3.3g PVP have the highest response (~58.4) to 100 ppm ethanol at 250^oC, which is about 9.4 times higher than that of the ZnO nanosheets obtained without PVP under the same test conditions (~6.2). Furthermore, the detection limit could reach the ppb level with the response of 1.17 to 500 ppb ethanol. The possible mechanism of the formed structures and the enhanced ethanol sensing properties are discussed systematically.