A new identification method using a proportional controller is proposed for automatic tuning of PID controller. A second-order plus time-delay (SOPTD) model is obtained by a least-squares method in the frequency domain. The suggested method can remove the restriction in applicability of previous methods so that it can be applied to any stable closed-loop response, regardless of the type of the transient such as overdamped, underdamped and inverse response. It is simple so that it does not need any complex techniques such as root finding or optimization method. With the obtained model parameters, the three parameters of PID controller can be calculated from the integral of the time-weighted absolute value of the error (ITAE) tuning rule on-line. Simulation study shows the robustness with respect to measurement noise and disturbance at the identification stage.