1D oxide semiconductors nanocrystals are considered to be highly efficient source for gas sensing due to their high sensitivity to surface chemical reaction, rapid response and high surface to volume ratios in comparison to thin film gas sensors. The effects of Mg doping on the structural, optical, morphological and gas sensing properties of ZnO nanobelts were investigated in this chapter. Toxic gases can affect human life and health even at levels of few parts per million and therefore, highly sensitive gas sensors are required. Since CO gas is tasteless, odorless and toxic and as such may not be easily detected by humans. Therefore, it is essential to develop a sensitive CO gas sensor to protect human's exposure to CO gas.