This paper presents some results on the relation between the dynamic properties of a subsystem and the eigenvalues of the overall system. Since the eigenvalues corresponding to the unreachable/unobservable modes of a subsystem is not affected by the dynamic properties of other subsystems, one can expect useful information from a measurement of the reachability/observability of a mode of a subsystem. In this paper, it is proven that when such a measurement is in a certain range, the position of the corresponding eigenvalue of the overall system is within certain boundary. The results can contribute to model assembly, model reduction and system synthesis. An example is presented to illustrate the application.