2008 Wenchuan earthquake has showed that failure of masonry walls is one of the major causes of material damage and loss of human life due to seismic events. Therefore, study of masonry walls seismic behavior and development of effective and practical retrofitting schemes is an urgent need. So this work presents an experimental program that investigates in-plane seismic behavior of common types of shale masonry walls before and after retrofitting using epoxy resin injection technique. In this experimental study, the primary objective was to evaluate the efficiency of this retrofitting method on masonry walls. The results of this study shows that using epoxy resin injection technique increased the in-plane maximum strength, the ductility, the initial stiffness value, the stiffness, and the energy dissipated during all the loading stages, and decreased the ductility factor.