Abstract This research is to study the seismic capacity of building taking account of the effects of soil- structure interaction. A 14-storeys reinforced concrete building was selected in this study. The building was modeled as slab-column frame located in Bangkok area where the site condition is soft clay, and it has not been designed for earthquake effect. In this study, nonlinear dynamic analysis was employed. The foundation was modeled as flexible foundation. The stiffness of the piles was varied to take into account of the flexibility of the piles. It was found that the seismic capacity of building with flexible foundation was less than that of building with fixed foundation, particularly when the soil was very soft. In addition, the seismic damage of structure with flexible foundation was greater than that of fixed base building.