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