A plain nano-crystal diamond(NCD) film with diameter of 5 cm has been prepared on commercial mirror-polished Si substrate by microwave plasma chemical vapor deposition(CVD) system in mixture gases of H_2,CH_4 and O_2.According to the observation of SEM picture and micro-Raman spectra the morphology and structure are analyzed.It is found that the film with high concentration of sp~(3+) has uniform crystal particle size,and the average size of diamond particles is about 150 nm.The etching mechanism by the mixture gases of H_2-O_2 is discussed through compared with SEM picture of micro-crystal diamond film deposited in the same condition and longer time.It is shown that the smooth substrate is important for the etching effect of O_2.O_2 with lower content compared with H_2 shows key effect during the beginning period of diamond film deposition.The influence gradually decreases when the grow substrate becomes rough.This work might benefit to the applications of diamond film such as surface acoustic wave device or infrared transmission coating film.