Under the fast-developing carbon trading market, the construction industry needs to mitigate carbon emissions from construction processes. Among various construction processes, tunnel construction produces a significant amount of carbon emissions, since it utilizes various types of high energy-consuming equipment. In order to identify and mitigate such carbon emissions of a tunneling project, it is required to reliably estimate carbon footprints of a tunneling project in the pre-planning phase. This paper presents the methodology for estimating the carbon footprints generated during tunnel construction processes using the collaborative tunneling simulation. A case study using this methodology shows that carbon footprints from a utility tunnel construction are significant compared with those from a building construction. In addition, the assessment of carbon footprints of the case study identifies the opportunities to mitigate such impact by supporting decision-making on equipment and operation plans in the planning phase and providing a control target level of carbon footprints in the execution phase. Copyright ASCE 2010.