The Gibbs free energies of formation of Nb(CO) and Nb(CN) were calculated using ab initio simulation. Phase stability diagrams of the Nb2O5-NbO2-Nb(CO)-NbC/Nb(CN) system were constructed with respect to carbon activity, oxygen pressure, and the characteristics of solution formation. The constructed diagrams suggest that Nb2O5 can be reduced without the formation of NbO owing to the formation of an Nb(CO) solid solution phase. The Nb(CO) phase might not be formed under a high partial pressure of nitrogen. This study demonstrates the absences of NbO and Nb(CO) during, respectively, the syntheses of NbC and Nb(CN) from Nb2O5. The constructed stability diagrams could also represent the conditions for syntheses of niobium carbide or stoichiometric niobium carbonitride by the carbothermal reduction of Nb2O5 under argon or nitrogen.