The robustness of solutions to the problem of pole placement obtained by the frequency–domain approach is examined. A more intuitive and general criterion that is necessary and sufficient for guaranteeing the located region of system poles is obtained. This new result is presented for all of open, closed or united regions and is applicable to both the continuous–time and to the discrete–time cases. The parumetrization of all admissible pole–region placement controllers is also introduced. In addition, a simplified criterion is provided to facilitate the analysis and synthesis of complex and high–order systems. When the system poles are requested to locate only in the left half–plane or the unit circle, the proposed result would become a similar but evidently less conservative condition of the stability robustness.