Znic oxide nanostructures have attracted much attention in gas sensing technology because of their high sensitivity to the chemical environments. In this research, we synthesized flake-like ZnO nanocrystals by chemical bath deposition method using znic acetate as the precursor and methanol as the solvent. The study on CO sensing by the ZnO nanoflake thin film were conducted under vacuum and atmosphere conditions at relative low sensing temperatures. The microstructure of ZnO nanoflake thin films was examined by using X-ray diffraction (XRD), scanning electrion microscopy (SEM), transmission electron microscope (TEM), and the surface chemical composition was analyzed by electron spectroscopy for chemical analysis technique (ESCA). The CO sensing study under vacuum shows that the electrical current increases when ZnO nanoflake thin films which was deposited on the Si substrate in 0.15 M znic acetate solution and calcined 350oC. The ZnO thin film has the best CO sensing performance at 100oC with a good reproducibility. At 200oC, the surface of ZnO nanoflake is reduced to metallic Zn. Under the atmosphere condition, the sensing response increase with the CO gas concentration. The best CO sensing performance was found to be at 200oC with a detection limit of 2500 ppm. After the CO sensing test in atmosphere, microstructure and chemical properties of the ZnO nanoflake thin films are intact according to XRD and ESCA analyses after the CO sensing test, and thus the sensing performance is highly reproducible in atmosphere.