This paper considers environmental city design using land use scenarios and large-scale traffic simulation. Low Carbon City (LCC) can be achieved by combining appropriate land use and transportation. We simulate the possible low carbon city by combining spatially explicit land use equilibrium (LUE) model and agent-based traffic model. First, land use scenarios of a city with different urban forms (compact and dispersed etc.) are created using the LUE model. Then the corresponding transportation is projected under each urban form with a large-scale traffic simulator for a case study city (Yokohama) in Japan. We also simulate the current traffic using the detailed person-trip data. Finally, we analyze the relationship between the urban form and the resulting CO2 emission both from land use and transportation are estimated. The proposed method can be a useful tool for urban planners to test some land use and transportation policies for designing sustainable cities.