In the fast developing technology of solid freeform fabrication, it remains a challenge to fabricate a heterogeneous object by layered manufacturing because conventional CAD systems and STL databases do not contain material and manufacturing information. This paper presents a novel approach to layered manufacturing of heterogeneous objects. A new processing algorithm based on the Extensible Markup Language (XML) format is being developed to process heterogeneous objects for layered manufacturing. The model database containing geometry, topology, material and manufacturing information is described by XML structural elements with the hierarchy analogized to the TREE data structure of the Boundary Representation (B-Rep) and/or the Constructive Solid Geometry (CSG) model. A process model for layered manufacturing is formulated based on the XML format for the fabrication of heterogeneous objects. A case study is presented to demonstrate the process algorithm for representing a multimaterial B-Rep model in the context of Three Dimensional Printing (3DP) technology. Specifically, the case study will show the conversion of a solid model to XML representation, the material assignment, the slicing, and the machine toolpath generation. The paper also presents a brief survey of technological application of XML and associated technologies.