Abstract
Petri nets represent a powerful tool for modeling the discrete event systems. The Petri net markings correspond to the system states. The infinity of the marking set means that the Petri net is unbounded and this may be the sign of an incorrect system model. In that case instead of the reachability set and the reachability graph the coverability set and the coverability multigraph can be used to represent the Petri net state space. A systematic way of building the notion of the coverability set and coverability multigraph based on the notion of the ω -marking is given in the paper. Algorithm for its construction is introduced. Then the use of the coverability multigraph for the analysis of several properties of the unbounded Petri nets is described.