Many researches of numerical and experimental studies have been reported in recent literature about the beneficial effects of infill walls on the seismic response of RC frames. A series of experiments have been performed in order to examine the impact of a thin layer of as Engineered Cementitious Composites, or ECC for retrofitting infilled RC frames. In the present study a numerical analysis is carried out on a one bay one story RC frame with unretrofitted and retrofitted masonry infill with ECC under in-plane loading.In that model a four-node shell element was used for the concrete, infill panel and ECC. The interaction between concrete and infill panel was modeled as well as the interaction between infill panel and ECC. Nonlinearities of the concrete, infill panel, steel and ECC were considered. The modeling was calibrated based on experimental results and then a parametric study was carried out, taking into account variation of thickness of ECC. The obtained results indicate that the proposed ECC- strengthening technique can effectively increase the lateral strength, energy absorption capacity of the infilled frame and the numerical simulations can predict load-displacement curves and crack propagation process with good agreement with experimental data