Abstract: Nanocrystalline diamond films were deposited by microwave plasma chemical vapor deposition method using Ar/H₂/CH₄gas mixtures.In order to explore the best growth conditions of diamond with small grain size,grains with different sizes were obtained by modifying gas flow ratio and power.The crystalline sizes and surface morphologies were characterized by X-ray diffraction and scanning electron microscopy.The qualities and residual stress were analyzed by Raman spectroscopy.The results show that the concentration ratio of argon and hydrogen has a greater influence on the grain size,relative to the factors of methane concentration and power.With the increase of the concentration ratio of argon and hydrogen,the grain sizes was decreased,however,the residual tensile stress influenced by the shift of the Raman G-peak was increased.Besides,the proportion of diamond to graphite phase was decreased firstly and then increased;meanwhile,diamond aggregates were smaller and surface roughness became better.