In this work, we report a new process for synthesis of nanocrystalline diamond films by using a small amount of simultaneous O-2 and N-2 addition into conventional CH4/H-2 mixtures. The nanocrystalline diamond samples were grown in a 5 kW microwave plasma assisted chemical vapor deposition system on large silicon wafers of 5.08 cm in diameter. Depending on the amount of O-2 and N-2 addition, the growth rates were in the range from 2.5 to 3.5 mu m/h. The samples were characterized by scanning electron microscopy, X-ray diffraction and micro-Raman spectroscopy. Our work demonstrates that the morphology, microstructure, grain size, crystalline quality and growth rate of nanocrystalline diamond films can be tailored by simply adjusting the amount of O-2 and N-2 addition, and with increasing the ratio of O-2/N-2 addition, the crystalline quality of the nanocrystalline diamond films is significantly enhanced, while the average grain size increases only slightly from 31 to 45 nm. This new process offers a simple way to tailor the growth of large-area uniform nanocrystalline diamond films of high growth rates and variable microstructures for different applications of nanocrystalline diamond films. (C) 2007 Elsevier B.V. All rights reserved.