Part flow and tool flow were the two mainly aspects of the workflow in flexible manufacturing system. During the past decades part flow deadlock was much more considered than that of tool flow. Tool flow deadlock problem in the condition that the production procedure can be interrupted was studied in this paper. The model of machine process based on the Petri net was set up and the definition of tool flow deadlock was proposed. Necessary and sufficient conditions for deadlock and potential deadlock were discussed according to graph theory method. A case was illustrated and showed that this method was to be correct and effective.