This paper studies the impact of hardware/software partitioning upon the testing cost of codesign systems. The testability of a hierarchical specification is discussed and an estimate proposed to evaluate the cost of system testing. It depends on cost values of hardware/software testing for each unit-level component. These values are provided by a mutation-testing approach applied to both software and hardware implementations. Results show that this approach provides a new helpful partitioning criterion which can be used together with other already known criteria. A case study provided by A?ROSPATIALE illustrates this testing cost oriented partitioning.