Abstract: Polycrystalline diamond films were prepared by using H2-CH4 as the source gas in a newly developed ellipsoidal cavity type MPCVD reactor. The influence of CH4 concentration on the diamond growth and quality was studied in this paper. On the basis of the above research, large area optical grade free-standing diamond film was prepared. Surface and cross-sectional morphology as well as the quality of the diamond films were examined by scanning electronic microscopy and Raman spectroscopy. Furthermore, the infrared transmittance of the samples was measured by Fourier transform spectrometry. The results show that the growth rate of the diamond films increases with the increase of CH4 concentration. However, when the CH4 concentration is increased to certain level, the diamond growth rate will no longer increase. High quality diamond films can be prepared when the CH4 concentration is between 0.5%-2%. The results prove that high quality optical grade diamond films can be obtained by this newly developed ellipsoidal cavity type MPCVD reactor.