In order to obtain diamond thin films of high quality and favorable electronic properties, we have examined the role of Xe addition to the microwave plasma-assisted {CH4 + H-2} chemical vapor deposition (CVD) for diamond thin films growth. To make clear effects of Xe addition, we compared the results with those of Ar addition. Effects of Xe addition were evident in the increased growth rate (about 50% increase for 1%-Xe), without degradation of the crystallinity, and in the marked improvement in the transconductance of diamond metal-insulator-semiconductor transistors (MISFETs). Based on the results of the plasma emission spectra, Raman shift and microwave plasma impedance measurements, the prominent effects of Xe addition were attributed to its lower excitation energy, which reduces plasma temperature and it is sufficient for the dissociation of atomic hydrogen and CH3 (precursors for diamond crystallization) but not CH? radicals.