As atoms migrate along void surface and grain-boundary driven by various thermodynamic forces, the grain-boundary void changes its shape and volume. A rounded void may become unstable and collapse. The void instability is an outcome of the competition between the variation in the elastic energy stored in the stressed solid, the void surface and grain-boundary energies. In this article, the instability conditions and the equilibrium shape of a gas-filled grain-boundary void are first determined, and then an explicit expression for the shrinkage rate is derived as a function of the void spacing, the applied stress, the internal pressure built up by the gas filled in the void as well as relevant material parameters. (C) 2004 Kluwer Academic Publishers.