The temperature vs. gas composition phase diagram of solid carbon deposition in atmospheric oxyacetylene flame is calculated. The phase diagram, which is composed of a diamond growth region, a gas phase region, and a nondiamond carbon growth region, is identical with the reported experimental data. So the suitable ranges of substrate temperature and O2=C2H2 ratio for diamond synthesis in oxyacetylene flames can be theoretically predicted. When the substrate temperature is between 1000K and 1250K, the corresponding O2=C2H2 ratio range is about between 0:8 and 1:1. The suitable range of substrate temperature is widest for O2=C2H2 ratio close to unity and is narrowed rapidly when the flow ratio deviates from unity.