Diamond films grown in (A)CO/H2 and (B)CO/O2/H2 systems at substrate temperatures (Ts) between 403 and 1023 K were characterized by x‐ray diffraction, Raman spectroscopy, cathodoluminescence, and scanning electron microscopy. A large amount of polyacetylene inclusion occurred in the (A)CO/H2 system on reducing Ts, resulting in worsening of the diamond crystallinity (FWHM of the diamond Raman peak broadened from 6.4 to 19.5 cm-1 when Ts was decreased from 1023 to 403 K). On the contrary, polyacetylene inclusion was significantly suppressed in the (B)CO/O2/H2 system, and high quality diamond films (FWHM=4.0–4.1 cm-1) close to natural diamond (FWHM=2.6–3.0 cm-1) were obtained between 684 and 1023 K. Though there was a little deterioration of crystallinity at 403 K, the obtained film still had good crystallinity (FWHM=10.2 cm-1) compatible with conventional chemical vapor deposition diamond films. The presence of a large amount of atomic hydrogen, atomic oxygen, O2, and OH contributed to suppression of polyacetylene formation on a growth surface and promoted cleaning of deposited amorphous phases. These species provided the best condition for selective growth of pure diamond of good crystallinity in the (B)CO/O2/H2 system even at low temperature (∼403 K), where impurities are likely to be involved. Films grown in the (B)CO/O2/H2 system were characterized as large and well‐defined crystallites of octahedral forms emitting intensive blue CL at 440 nm. The actual activation energy (7.0 kcal/mol) for homoepitaxial diamond growth was obtained using the (B)CO/O2/H2 system, and was in good agreement with previous quantum chemical calculations (6.33 kcal/mol) based on the methyl precursor model. Finally, the (B)CO/O2<- /inf>/H2 system was suggested to be one of the most promising gas combinations for low temperature growth of high quality diamond.