A workflow management system is a software system to support accurate execution, control and management of business processes. It has been known that the system automatically executes complex processes and effectively manages them to improve the productivities. However though existing systems assure the automatic execution of an individual process, there is a room for enhancement from the view of efficient execution of all the processes. In this paper, we propose a method of executing business processes efficiently by introducing the PERT/CPM techniques in the workflow management systems. We first consider the differences between workflow process models and PERT/CPM models, and then develop a method of calculating the critical path and slack time in workflow processes. This leads us to develop a dispatching rule that can guide task performers to prioritize their tasks to increase the efficiency of ail the processes. We have carried out a set of simulation experiments and analyzed the results to demonstrate the effectiveness of the proposed method.