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Carbon dioxide (CO2) emissions mitigation from transportation infrastructure construction activities is regarded as one of the potential pathways to deal with climate change. Aiming to assess the magnitude of CO2 emissions associated with freeway infrastructure construction activities and identify primary emissions sources in mountainous area, this study firstly assessed life cycle CO2 emissions of an entire freeway project in mountainous area in China, including pavement, bridge, tunnel, intersection, traffic safety facilities and temporary works into the system boundary. The results show that CO2 emissions related to road segment are one magnitude lower than that of bridge and tunnel. In addition, negligence of intersection, traffic safety facilities and temporary works could underestimate 4% of total CO2 emissions of the freeway project. CO2 emissions distribution of the different types of materials and equipment distribute as the similar Pareto Principle, which reminds that definition of system boundary could focus on the few of inputs that matter on the aspects of data limited, rough estimation and comprehensive understanding of environmental impact. Findings of this study could offer transport agencies more comprehensive references to understand the contribution of freeway infrastructure construction activities in the mountainous area to climate change, and provide decision makers insights to take measures in the planning phase, particularly in the construction phase in terms of equipment selection, construction design, and construction materials design.