The conventional torch flame of C2H2/O2 mixture can grow diamond film at fast rates. But it suffers from the following problems: small area, non-uniform growth, and ill re-produciblity. Considering the effects of momentumn, heat and mass transfer, we proposed to study diamond growth by flat flame method. Our results indicated that, due to large burning velocity of C2H2/O2 mixture, dilution in nitrogen is required for reasonable amount of gas consumption. Although the possibility of diamond growth by flat flame method was illustrated, the dilution effect reduced the growth rate of diamond significantly and deteriorated the crystal qualities. The possible reasons will be discussed. The effects of bias of flame deposition will also be discussed.