A diffuse-interface field approach is developed to model the behavior of long-time phase separation in strained cubic alloys. Contributions to the free energy of the system from bulk chemical mixing energy, concentration gradient energy and coherent misfit strain energy are incorporated. Special attention is focused on the effect of stress caused by lattice misfit, because it alters not only diffusion barrier but also kinetic constants. The resulting non-linear diffusion equation is solved by semi-implicit spectrum methods in three dimensions. Numerical results indicate that stress field dominates the late-stage phase separation, and various ordered structures can be formed in the path towards equilibrium. (C) 2002 Elsevier Science B.V. All rights reserved.