We worked out the excess energies for bulk InxGa1-xN and InxGa1-xN thin films on GaN and InN in order to investigate their thermodynamic stabilities. It has been found that the excess energy maximum shifted toward x similar to 0.80 for InGaN/GaN and x similar to 0.10 for InCaN/InN due to the lattice constraint in contrast with x similar to 0.50 for bulk. Moreover, it has been revealed that the excess energy for InGaN/GaN is larger than that for bulk at x > 0.65. This suggests that In-rich films are less stable on GaN than bulk state. These results indicate that the lattice constraint has a significant influence on thermodynamic stabilities of thin films.