In order to investigate the growth mechanism of diamond thin films, theinsituoptical emission spectra of direct current (dc) arc discharge plasma, including the spatial distributions and different CH4/H2ratios, have been measured during the growth processes of diamond thin films prepared by the dc arc discharge plasma chemical vapor deposition method. The results show that there are a great number of atomic hydrogens in the arc discharge plasma. This is the key factor for the growth of diamond films with a high rate and high quality. In addition, the effects of the CH4/H2ratio on the quality of diamond films are discussed in detail in this letter.