The dependence of the microstructures of the {111} surfaces of diamond crystals synthesized in the H2‐CH4‐O2system on the oxygen‐carbon ratio (O/C) has been studied to elucidate the mechanism of the crystal growth. On the {111} surfaces of diamond crystals synthesized in high O/C (≥0.8, in our synthesized conditions), the two‐dimensional nucleation rate is suppressed and the two‐dimensional crystal growth rate is promoted. The Raman spectrum of the diamond crystals synthesized in high O/C shows that the half width at half maximum and the peak position are equivalent to those of natural diamond crystals.