The thickness of i-diamond in the diamond-based metal-insulator-semiconductor (MIS) gas sensor has been found to be a major factor in determining the hydrogen gas sensing performance. This study reveals that the reduction of the i-diamond thickness in the sensor structure provides a larger gas sensitivity (or larger change in electrical parameters), larger response rate (or faster response), and smaller adsorption time constant upon exposure to hydrogen gas. However, the reduction of the i-diamond in the device reduces the dynamic range of hydrogen gas sensitivity. Structural and electrical properties of the plasma-enhanced chemical vapor deposition diamond films in the sensor were determined by using scanning electron microscopy, Raman spectroscopy, and current-voltage characterization tools, and their effects on gas sensing performance of the sensor were analyzed.