In this paper a Petri net based modeling approach for flexible process planning is presented. The proposed model can represent alternative operations and operation sequences for a given part. It is possible to observe the precedence constraints on the Petri net and all possible process plans can be generated by applying reachability analysis. Various heuristic criteria can be used to find efficient process plans among high volume of alternatives. Since the proposed model offers a considerable flexibility for process planning task, it can be used as a process planning module in an integrated manufacturing system for making effective real-time decisions.