A relay based on-line automatic tuning method for PI controllers for stable processes is presented. In the proposed method, prior to controller re-tuning a relay in tandem with the controller and plant induces limit cycle oscillations. Based on the limit cycle measurements, a first order plus dead time (FOPDT) model of the process dynamics is obtained. Simple tuning rules based on ISTE performance criterion and the first order model are developed. The controller settings may be re-tuned non-iteratively to achieve enhanced performance without disrupting closed loop control. A number of simulation examples are given to illustrate the potential advantages of the proposed on-line tuning method. (C) 2005 Elsevier Ltd. All rights reserved.