The phase diagrams of diamond growth from CH4/H2, CO/H2 gases and oxyacetylene flames are calculated by non-equilibrium thermodynamic coupling model in the paper. These phase diagrams are different from those derived from classical equilibrium thermodynamic theory. A diamond growth region, which is the thermodynamic basis of diamond synthesis, exists in each calculated phase diagram. The diamond growth region reflects the roles of those activated particles such as super equilibrium atomic hydrogen during diamond deposition. The calculated phase diagrams are agreement with many reported experimental results, and they will be of favor of theoretical and experimental studies of diamond growth from gas mixtures.