We propose three characterization theorems of / -diagnosable systems under the comparison model. By these characterization theorems we present some properties of / -diagnosable systems. Furthermore, for a given network system, we introduce a new method to determine a range from min to max conveniently, in which the system is at least min / min -diagnosable and at most max / max -diagnosable. By applying our results to some well-known networks such as -dimensional hypercube, 2 (3 ) mesh, and permutation star graph, we figure out their / -diagnosability.