Abstract: Nano-crystalline diamond (NCD) films were deposited on single crystal Si (100) with Ar+CH4+H2 gas mixtures as gas source using hot filament chemical vapor deposition method with different Ar volume fractions.The surface morphology,microstructure and residual stress of the nano-crystalline diamond films were analyzed by using SEM,AFM,XRD and Raman spectrometry.The results show that the grain sizes decreasd to nanometer dimensions with the increase of Ar volume fraction.The film residual stress transformed from the tensile stress to compressive stress due to the grain refining.The residual compressive stress increased first and then decreased as increasing the Ar volume fraction.It was found that NCD films with an average grain size of 54 nm and a roughness of about 14.7 nm had been successfully synthesized in the poor-hydrogen atmosphere with 98%Ar.