Delay in a software project may result in the loss of a market opportunity or the postponement of a dependent project. Therefore, software project managers take various steps to ensure that their project is completed on time, such as adding new members to the project team. However, adding new manpower to a delayed project may cause a negative impact on the team's productivity due to assimilation time, training overhead and communication overhead. Consequently, project managers have difficulty in making the decision on whether or not to add new members to the team. Thus, this research aims to examine whether a significant schedule improvement can be achieved with consideration of the new manpower's capabilities, skills and experience. A System Dynamics Model is proposed to simulate the behaviour of a project's progress when new members are added. The proposed model was evaluated through experiments using two types of case studies. The results of the experiments indicate that a significant schedule improvement of a late project can be achieved if people with certain levels of personnel factors are added to the project.