Changes in user requirements or project personnel occur frequently during project execution particularly in long-term and large-size projects. We need a tool which can estimate the effects of changing conditions to effectively manage the project. This paper proposes a simulation method for dynamic project performance in terms of effort, schedule, and defect density changes in a dynamic project environment by combining COCOMO II with system dynamics. We apply expert judgment technique to overcome the lack of empirical data on the effects of dynamic project environment. We develop a simulation tool (available on the authors' website) which has model adjustment parameters to reflect experts estimation on project characteristics. The simulation experiment on a military application development project demonstrates that the developed model can show the behavioral characteristics of a project suffering unanticipated and uncontrolled requirements creep. This helps project managers understand interactions between project factors and proactively evaluate and control the effects of dynamic project environment. (C) 2008 Elsevier B.V. All rights reserved.