The surface barriers at the neck of nano-ZnO gas elements are expressed in terms of the electrical potential inside a cylinder, and then the resistance, the sensitivity, and the grain-size effect are discussed. It is shown that the sensing property of nano-ZnO gas elements is influenced by the microstructural features, such as the grain size, the geometry, and the connectivity between grains. The difference between the neck-controlled sensitivity and the neck-grain-boundary-controlled sensitivity is large when the width of the depletion layer at the neck in air is comparable with the neck radius. It implies that although the grain-boundary resistance may be much smaller than the neck resistance it cannot be neglected. It is suggested that the decreasing of the ratio between the numbers of grain boundaries and necks is a possible approach to the development of nano-ZnO gas sensors with a high sensitivity.