Abstract: The difference between activity-on-arrow network and activity-on-node network is the symbol exchange，which brings the variations on the time index calculation and critical path． These variations are caused by the dummy activity in activity- on-arrow network; hence，the main difference between activity-on-arrow network and activity-on-node network lies in whether the dummy activity exists in activity-on-arrow network． Taking an example，the differences are caused by the dummy activity． Dummy activity is important in the application of network planning．

Key words: activity-on-node network, activity-on-arrow network, dummy activity

摘要： 单代号网络图与双代号网络图区别的主要形式是其符号含义的互换。由于符号含义互换带来它们之间某些 时间参数计算和关键线路确定的方法不同，但是这些不同都是当双代号网络图存在虚工作时引起的。所以单双代 号网络图除符号含义不同之外，其时间参数计算的实质区别取决于双代号网络图中有无虚工作。针对网络计划一 实例分析了虚工作造成的实质性区别，说明了这种实质性的区别在网络计划应用中的重要性。

关键词: 单代号网络图, 双代号网络图, 虚工作