Diamond films were deposited in electron assisted chemical vapor deposition (EACVD) reactor using two source mixtures of CH4-H2 and C2H5OH-H2

respectively. The plasma gas composition during diamond growing was investigated in situ using optical emission spectroscopy(OES). In two cases of C2H2OH-H2 and CH4-H2 plasma

it was shown that CH and CH+ were all important precursor species in the diamond deposition reaction while the yields of poor diamond films corresponded to the presence of the C2 emission line. The difference between these two cases was that some oxygen-containing species (CH2O

CHO and O2) were detected in the C2mOH-H2 plasma. The presence of these products may maintain the quality of the deposited diamond films while increasing carbon source concentration

and the growth rate was thus enhanced. These results imply that the increase in the growth rate of diamond film using C2H5OH-H2 mixture is primarily due to a change in gas phase environment.