Diamond films grown in the microwave plasmas of CO(7–8%)-O2(0–2.2%)-H2 systems in the range of 130–750°C were characterized by scanning electron microscopy, Raman spectroscopy, and cathodoluminescence (CL) studies. The films grown in the CO-O2-H2 system had much better crystallinity than those grown in the CO-H2 system. This was because oxygen extremely purified diamond films by suppressing polyacetylene inclusion, and prohibited the vacancy formation in the crystallites. These oxygen functions have indicated the possibility that high quality diamond films (FWI-tM of the diamond Raman peak =4.0–4. lcm−1) close to natural diamond (FWHM=3.0cm−1) were obtained in the CO(8%)-O2(2.2%)-H2 system between 400 and 750°C. Though crystallinity deterioration occurred at 130°C, the obtained film (FWHM=10.2cm−1) in the CO(8%)-O2(2.2%)-H 2 system was of good crystallinity comparable to those (FWHM=7–21cm−1) grown by conventional CVD processes and gas systems between 590 and 1327°C. The CO-O2-H2 microwave plasma was concluded to be one of the best environment for the low temperature growth of highly purified diamond films of good crystallinity.