The main focus of this study is to investigate the NO2 gas-sensing properties of HfO2 thin films, taking into the account the importance of the annealing environment and temperature. HfO2 films were produced with 10 nm and 20 nm thicknesses by atomic layer deposition (ALD). They were annealed under N-2 and O-2 environment with rapid thermal annealing system at two different temperatures. It was found that the NO2 responses of annealed in N-2 environment were higher than annealed in O-2 environment. The response of 10 nm thickness HfO2 thin film was calculated 22% for N-2 environment whereas, it was calculated 18% for O-2 environment. The highest gas response is obtained for 10 nm thickness of HfO2 thin film annealed in N-2 environment. The performance of the HfO2 thin films reveals that the gas responses rely on annealing temperature as well as annealing atmosphere. The linear dependences of the sensitivity were also observed as the NO2 gas concentrations varied from 10 ppm to 30 ppm in logarithmic forms, indicating that HfO2 thin films offer promising potential for future practical applications in monitoring the NO2 gas.