Abstract: Effects of different activities on the expected completion time of a project are not the same. Based on this fact, one of the most important issues in project management is to determine important activities and the amount of effort that should be assigned to control them, thereby completing the project successfully. In this regard, several classical criticality indexes, including activity criticality probability (ACP), activity cruciality index (ACI), and significance index(SI), are thoroughly compared and analyzed herein. Due to the shortcomings of all these indexes, a new one, namely activity importance index (VES), is adopted to measure the importance of project activities. Moreover, based on the fact that expending effort on activities and controlling them actively can result in a shorter mean duration of the project, this paper puts forward a thought that the project managers allocate their own energy according to the critical index of each activity. Finally, we develop a network planning software, C#-NETSIMU, using Visual C# and apply it to analysis of a case project. This study would help project managers for reasonable project control.