This paper proposes an agent-based simulation model to analyze the roles of mutual communications among members of a project team. The objective of the research is to uncover the characteristics communication efficiency in a large scale and short delivery project. In order to explore the better communication ways in project management, we develop an agent-based simulation model, in which we implement a manager-leaders group as communication agents. From intensive experiments, we have found that (1) the project productivity is proportional with the communication efficiency, (2) making a delivery period of large projects shorter is difficult comparing small projects, (3) it is effective to communicate about many issues in one chance of communication in a large scale project, and (4) in any size of projects, it is effective to take enough time for communication.