We propose a simple on-line process identification method for the automatic design of PID controllers. It can identify general time-invariant linear systems with time and frequency weighting, while retaining simplicity. We then use a model reduction method to tune PID controllers using usual tuning rules based on the first or second order plus time delay model. It can efficiently incorporate initial value problems of previous identification methods for autotuning by assigning zero weight to the initial state. The proposed method shows an acceptable robustness to measurement noises and disturbances.