We propose a new and simple on-line process identification method for the automatic tuning of the PID controller. A least-squares method is used to obtain the model from measured process outputs and inputs without any structural information. Thus, any kind of test signal generators such as a relay, a P controller, the controller itself, pulse and step signal generators, etc can be utilized only if they contain sufficient information because it requires only the measured process output and the controller output. General time-invariant linear systems including open-loop stable, nonminimum phase, unstable and integrating processes can be identified effectively by the proposed identification method. We then use a model reduction method to tune the PID controller using usual tuning rules based on the first- or second-order plus time-delay model.