To achieve the data exchange between heterogeneous systems introduced by interoperability, the algorithms for model mapping in STEP are presented, which provide a methodology to the integration of CAx systems based on various application protocols of STEP. A formal definition of STEP/EXPRESS data model is introduced firstly. Based on this definition, two kinds of model mapping techniques for STEP, i.e. the static mapping and the dynamic mapping, along with the implementation by using EXPRESS-X mapping language are elaborated. Finally,a typical application of this techique in the open environment is addressed.Being independent of any specified system,the model mapping techiques presented in this paper achieve transparent integration beween heterogeneous systems,which reduce the cost of information integration in system development.