Abstract: An object-oriented model of building is presented in this paper where a series of decomposition among objects is carried out. A virtual face and a datum point which embody basic semantics requirements are two low-level objects. They hold not only geometric data, but also topology relation. The first is used to construct wall faces, spaces and solid primitives and the second constructs wall line and corner point of a space. The paper also shows that in an object-oriented approach topology connection among objects is more flexible and less dependent than in the case of structurebased model.