<p>Diamond films have excellent mechanical characteristics, such as high hardness and low friction, but have rough surfaces. Therefore, we developed a method for synthesizing diamond films while modulating carbon gas concentration using microwave plasma chemical vapor deposition. Then, the effects of concentration modulation on the mechanical properties of smooth diamond films were examined. A concentration-modulated diamond (CMD) film with a surface roughness (Sz) of 0.2&#xa0;µm was synthesized by modulating the methane concentration from 1% to 10%. The diamond films synthesized by modulating CH₄ concentrations were classified as nanocrystalline diamond (NCD) films by Raman spectroscopy. The hardness of the CMD film ranged from 43.8 to 61.1&#xa0;GPa, exhibiting higher values than those of NCD synthesized at 10%. In friction testing with Al₂O₃, the friction coefficient of the CMD film was below 0.1. The specific wear rate of the counterpart material subjected to dry conditions was 0.09 × 10⁻⁶&#xa0;mm³/Nm, demonstrating a reduction of over 90% compared with conventional diamond films.</p>