Abstract: The liveness problem for discrete manufacturing/assembly systems is studied first in this paper. We develop a Petri net model for processings of jobs in a manufacturing/assembly system. By analysis of the Petri net model, two kinds of structural objects which can lead to system deadlock and the liveness characteristics of the system are obtained. We then present our deadlock- avoidance Petri net controller for a class of manufacturing/assembly systems. This controller can be implemented easily and places less restrictive requirement on systems. And the controlled system can be modeled by a Petri net. For a general manufacturing/assembly system, we present a policy which can guarantee that the controlled system is live.