A monitoring procedure is proposed to identify on-line the maximum closed-loop log modulus, L-c/max, for multiloop control systems. L-c,L-max is considered because it measures the robustness of the control systems and thereby can indicate the appropriateness of controller design with respect to the current operating conditions. The proposed method identifies L-c,L-max via relay feedback experiments without requiring a priori information of process conditions. This procedure is applied to both linear systems and nonlinear systems. The results show that the proposed method is efficient and accurate in on-line identification of L-c/max.