Diamond films have been synthesized by flame combustion using a mixture of commercial acetylene and oxygen gas, but the flame combustion conditions were unstable, owing to the use of commercial acetylene. This is due to the variable purity of commercial acetylene that includes dissolved impurities. Therefore, in this study, high-purity acetylene was used to synthesize diamond films; it was obtained through special dissolution of pure acetylene of stable and high purity. To obtain good quality diamond films and to achieve good adhesion, diamond films were synthesized by stable flame combustion using a mixture of highpurity acetylene and oxygen gas with nitrogen gas added as the diamond promotion agent; nitrogen flow rate was carefully varied in the range of 0.333∼0.667 cm3/s. According to the results, the optimal nitrogen flow rate was 0.500 cm3/s, and good quality diamond films were synthesized. The synthesized crystallites also showed almost uniform size and high density. Moreover, as nitrogen flow rate was further increased or decreased from 0.500 cm3/s, the diamond quality of the synthesized films was reduced. The delamination of films synthesized with varying the nitrogen flow rates was prevented at a very high probability. In particular, the delamination of the films synthesized at a nitrogen flow rate of 0.500 cm3/s was completely prevented. To investigate the effects of nitrogen addition on the synthesized diamond films and film delamination, the nitrogen density of synthesized films was analyzed. The nitrogen density of the films synthesized with nitrogen addition was higher than that of the films synthesized without nitrogen addition. The nitrogen density of the synthesized films was increased by nitrogen addition. Thus, nitrogen addition to high-purity acetylene affects diamond synthesis. Moreover, the crystallite morphology of synthesized diamond films can be changed by varying the nitrogen flow rate.