The conventional off-line approach to supervisor synthesis for discrete-event dynamic systems contains critical drawbacks in building overall automats models and calculating the complete control policy for these models. To resolve the complexity of this off-line approach, the supervisory control scheme based on limited lookahead control policies was proposed by Chung et al. and improved by Kumar et al. recently. However, these methods do not guarantee the same closed-loop behavior as an optimal off-line solution. Therefore, we propose an on-line tracing supervisory control scheme founded on component discrete-event models, which ensures the same closed-loop language as an optimal off-line scheme. (C) 1999 Elsevier Science Ltd, All rights reserved.