A rapidly rising interest in core-shell particles stimulated by the great achievements of modern nanotechnology brings to the fore the development of simple, convenient and practically useful techniques to simulate and design the optical properties of the particles and their composites. We develop an approach to calculate the third-order optical nonlinear susceptibility of three-phase composites consisting of the core-shell nonspherical particles embedded in a host. The approach is based on calculation of the averaged internal electric fields of the phases. As an example, a composite system made of shape-distributed core-shell metal/dielectric ellipsoids is considered. (c) 2007 Elsevier B.V. All rights reserved.