In this study,the nanocrystalline diamond (NCD) films were prepared b y a DC arc plasma jet chemical vapor deposition (CVD) system. Nucleation and growth of the NCD films highly depend on the pretreatment process for substrates.The effects of diamond particles in different solvent systems on the nucleation of n anocrystalline diamond films are analyzed.The nucleation density of the NCD films decreases with the in creasing size of the diamond particles in the solvent systems.The solvent system contains dimethyl sulfoxide (DMSO) and us ing it to pretreat substrates can enhance nucleation density,and crystal nucleus becomes smaller and more intense.Final ly,the NCD films were grown for 60min after pretreatment in a mixed solution of acetone and dimethyl sulfoxide containing 5nm diamond nanoparticles.The films consist of diamond nanocrystals with 50－70nm diameter, and with good crystalline quality. The NCD films are suitable for the high-frequency surface acoustic wave devices and various optical windows.