Under the assistance of magnetic field, the electron cyclotron resonance effect generates a high density plasma in a large spaceregion This offers an beneficial condition for large area diamond films deposition. By using a mixture of methane (CH4) and hydrogen (H2), a diamond film of 5 cm diameter has been grown on silicon substrate under the pressure about 3 Torr which is considerably lower than the pressure in all other methods recently used. In view of the spacial extent of the plasma, we see that it is possible to further increase the area of deposited diamond film. The growth condition of diamond film in gas phase deposition under the pressure of 0.5 to 50 Torr has been discussed on the basis of analyzing the optical emission spectrum of the plasma.