This paper first defines social capacity of environmental management (SCEM) as the capacity that the whole society, composed of three social actors: government, firms and civil society, makes use of available capital assets (including natural capital, physical capital, financial capital, human capital, and social capital) to deal with environmental problems toward sustainable states through the learning process under the influence of actors' co-existence, inter-actor interactions and future uncertainty. Then, a structural equation model is applied to estimate the SCEM at city level in transport sector. Millennium Cities Database is adopted to measure environmental sustainability. Since it is difficult to collect capacityrelated data at city level, and it is also expected that capacity at nation level might influence that at city level, the capacity-related data at nation level contained in Environmental Sustainability Index is used to measure the influence of SCEM on the environmental sustainability. The effectiveness of the proposed model structure was empirically confirmed and the relationship between SCEM and environmental emissions was clarified. Some limitations of applying such macro-level data were also observed.