A relay feedback based method for tuning a PI/PID (proportional-integral- derivative) controller with an assessment of gain and phase margins is proposed. The proposed method estimates the gain and phase margins of an existing control system by a modified relay feedback scheme, where a delay element is embedded between the relay and controller. The gain and phase margins of a control system are used to assess the control performance. When the controller is found not in good performance, the proposed method is then applied to retune the PI/PID controller based on gain- and phase-margin specifications. Simulation results have shown that the proposed method is effective for processes with different kinds of dynamics and for multiloop systems.