Flat plate structures subjected to lateral load have less deformability than conventional moment frames, due to the brittle failure of plate-column connection. In the present study, parametric study using nonlinear finite element analysis was performed to investigate the deformability of flat plates. The numerical results show that as number of continuous spans increases, the deformability of flat plates considerably decreases. Therefore, existing experiments using sub-assemblages with 1 or 2 spans may overestimate the deformability of flat plates, and current design provisions based on the experiments may not be accurate in estimating the deformability. A design method estimating the deformability was developed on the basis of numerical results, and verified by comparison with existing experiment. In the proposed method, the effects of primary design parameters such as direct shear force, punching shear capacity, aspect ratio of connection, number of spans, and initial stiffness of plate can be considered.