Optical emission spectroscopy (OES) is used to diagnose the CH₄/H_2pl asma online during the synthesis of diamond film by microwave plasma chemical vapor deposition(MP -CVD).The effect of CH_{4concentration} on the concentration and distribution of radicals in C H₄/H_2plasma is studied.The diamond films deposited in different CH_{4concentrations} are in vestigated using Raman spectroscopy.The results show that the spectrum intensities of radicals of Hα,Hβ,Hγ,CH, C_2increase with the increase of CH_{4concentration},especially,that of radical C_2has notable improvement.The electron temperature of CH₄/H_2plasma increases with the increase of CH_{4concentration}.The spectrum space diagnosis results show that the distri bution of the radicals in the plasma ball is uneven,and the uniformity of C₂,CH radicals ten ds to poor with the increase of CH_{4concentration}.The measurement of deposition rate shows that only increasing the CH_{4concentration} is not useful for the growth rate of diamo n d films.The Raman spectra indicate that the diamond film deposited by lower CH_{4concentration} (0.8%) has better quality.