Diamond films were generally deposited by CVD method from a hydrocarbon as a carbon source diluted with an amount of hydrogen, while recent studies reveal that diamond can be deposited in rare gas based plasma with a small portion of oxygen. Therefore, in this paper, we have investigated the effect of diluent gas on diamond growth in order to clarify the deposition mechanism, especially the role of hydrogen. The parameter range of deposition for single phase of diamond was expanded by the addition of a small amount of hydrogen into CH4-O-2-Ar plasma. At smaller CH4 concentration, larger H-2/CH4 might be necessary for the single phase deposition of diamond. During the deposition in Ch(4)-O-2- (Ar/H-2) plasma, the emission intensity of H-beta line was almost constant in spite of H-2 concentration. Although much higher population of H could not be needed for diamond formation due to diamond growth in no H-2 addition plasma, a certain H density must be essential for the single phase deposition of diamond.