This study evaluates the impact of the vacant lots on the outdoor thermal environment and building heat load in the summer in an historical urban district using the 3D CAD-based thermal environment simulator. In this study, The spatial geometry and material data in 1979 and 2009 are collected and the 3D CAD model of the site scaled 1:500 is completed. As a result of numerical simulation, Heat Island Potential, which is the evaluation index of the sensible heat flux to the atmosphere, of the site where the adjacent site became vacant lots in 2009 is 20°C at a maximum higher than that in 1979. Furthermore, the heat loads of the buildings adjacent to vacant lots is about 1.2-1.4 times as large as that of other buildings that have the equivalent total floor area and heat insulation capacity. Therefore, the increase of the vacant lots affects the changes of the outdoor thermal environment in the site and the heat load of the buildings adjacent to the vacant lots.
