Critical path method(CPM) network is one of the most useful instruments for project management.The relation ship between the activity float and the path length,which shows the fundamental laws of an activity-on-arc CPM networks,is studied in this paper.Two equations about the length of the longest path from start node to arbitrary node and from arbitrary node to end node are given.Relative theories such as "Total Float Theory" are presented and demonstrated on the basis of the research above.The...