Thermodynamic analyses were carried out to understand influence of lattice constraint from GaN and InN substrates on relationship between solid composition x of InxGa1-xN films and input mole ratio R-ln (= P-ln(0)/(P-ln(0) + P-Ga(0)) where P-i(0) is the input partial pressure of element i) during molecular beam epitaxy. The calculated results suggest that compositional unstable region is found at small R-ln region for InGaN on InN while that for InGaN on GaN can be seen at large R-ln region at higher temperatures. This implies that InN-rich thin films are possible to form on InN substrate though it is difficult to form on GaN substrate. (C) 2003 Elsevier Science B.V. All rights reserved.