Alloy phase stability of zincblende Ga1-xMnxAs was investigated by using the first-principles full-potential linearized augmented plane-wave method and the cluster expansion method, within the local spin density approximation (LDA), generalized gradient approximation (GGA), and LDA + U. All approximations demonstrate that the system has a tendency to segregation and inherently favors clustering. Importantly, calculated lattice constant in the LDA + U is found to increase when the substitutional Mn composition increases, which appears to support recent experiments. (c) 2006 Elsevier B.V. All rights reserved.