This review article discusses the gas sensing properties of metal oxide nanostructures, focusing on their particle size, morphology, and doping effects. Metal oxide gas sensors are widely used due to their low cost, easy production, and compact size, but their performance is significantly influenced by the morphology and structure of the sensing materials. The "small size effect" is highlighted, where the sensitivity of sensors increases when the particle size is close to or less than twice the thickness of the space-charge layer. However, small nanoparticles can compactly sinter during film coating, hindering gas diffusion. To address this, various nanostructures such as porous nanotubes, nanospheres, and nanosheets have been developed, which offer larger surface areas and improved gas diffusion. Doping is another effective method to enhance gas sensing properties by reducing particle size and improving sensitivity. The article reviews the gas sensing mechanisms of n-type metal oxides, the device structures of metal oxide sensors, and the impact of nanostructures and doping on their performance. Finally, it discusses future directions, including the development of novel nanostructures, combining porous nanostructures with chromatographic techniques, and further investigation of gas sensing mechanisms using first principles.This review article discusses the gas sensing properties of metal oxide nanostructures, focusing on their particle size, morphology, and doping effects. Metal oxide gas sensors are widely used due to their low cost, easy production, and compact size, but their performance is significantly influenced by the morphology and structure of the sensing materials. The "small size effect" is highlighted, where the sensitivity of sensors increases when the particle size is close to or less than twice the thickness of the space-charge layer. However, small nanoparticles can compactly sinter during film coating, hindering gas diffusion. To address this, various nanostructures such as porous nanotubes, nanospheres, and nanosheets have been developed, which offer larger surface areas and improved gas diffusion. Doping is another effective method to enhance gas sensing properties by reducing particle size and improving sensitivity. The article reviews the gas sensing mechanisms of n-type metal oxides, the device structures of metal oxide sensors, and the impact of nanostructures and doping on their performance. Finally, it discusses future directions, including the development of novel nanostructures, combining porous nanostructures with chromatographic techniques, and further investigation of gas sensing mechanisms using first principles.