Abstract: In this paper, a test-based hardware/software partitioning approach for co-design specifications is presented. The testability of a hierarchical specified co-design system is discussed and an estimate is proposed to evaluate the system testing cost. It depends on the hardware/software testing cost values for each unit-level component. These values are provided by a mutation-test approach applied for testing both software and hardware unit-level implementations. Results have shown that this approach provides a new helpful partitioning criterion which can be used with other already known criteria. A real case study provided by Aerospatiale illustrates this testing cost oriented partitioning.