High quality diamond films have been rapidly synthesised by the DC arc discharge plasma CVD in a hydrogen-methane mixture gas. The properties of the films are tested and analyzed by scanning electron microscopy, X-ray diffraction and Raman spectra. In order to find out the growth mechanism of vapor deposited diamond, in situ optical emission spectra of plasma under practical growth conditions are measured. It is found that the key factor of rapid growth diamond film is the presence of a large number of atomic hydrogen in the plasma.