Abstarct: A significant number of unreinforced masonry (URM) buildings have been constructed in Iran. Due to the lack of design, poor quality of materials and construction, most of these buildings are vulnerable against earthquakes. Therefore, different seismic rehabilitation methods have always been considered by researchers. In addition to studying the retrofitting methods of these buildings before damage, it is necessary to evaluate the retrofitting methods after occurrence of earthquake. The aim of the present study was the investigation of damaged and un-damaged URM walls behavior, retrofitted by Engineered Cementitious Composites (ECC), through experimental and numerical methods. The present experimental research was carried out in two stages of constituent materials (masonry and ECC materials) testing and structural (URM wall specimens) testing. The goal of the material experiment was for numerical modeling and ensuring that their characteristics were consistent with the assumptions of the structural specimens. For structural testing, three half scale masonry walls including reference (un-retrofitted), undamaged and damaged retrofitted specimens were constructed and subjected to a constant gravity (0.1MPa) and cyclic lateral loads. The average thickness of the ECC laxyer for retrofitted undamaged and damaged walls was considered to be 15mm and 20mm, respectively. To connect the ECC laxyer to the wall foundation, steel rebar dowels were used. Experimental results indicated that ECC mortar improves the integrity between masonry units. This leads to remain the in-plane wall stability up to the end of the experiment. Reference and retrofitted specimen failure modes were bed joint sliding and rocking (flexural behavior), respectively. Shear strength and energy dissipation of both undamaged and damaged retrofitted specimens increased significantly rather than those of the reference specimen. The amount of shear strength and energy dissipation of the undamaged retrofitted specimen were 170% and 40%, respectively more than those of the reference specimen. This amount for the retrofitted damaged specimen was 330% and 115%, respectively more than those of the reference specimen. The ductility of the retrofitted specimens was not significantly different from that of the reference one. The difference between the initial stiffness of the reference specimen and the retrofitted undamaged and damaged specimens in the same drift of less than 0.01% was less than 20%. While in the same drift and with increasing displacement, the difference between the reference specimen stiffness and that of the retrofitted ones increased. This can be due to the coherence of the masonry units by the ECC laxyer and the wall stiffness return to the pre-damage state. In the numerical stage of the study, the effect of gravity load levels, the ECC laxyer thickness and one or double-side retrofitting on the URM wall lateral strength were investigated. The numerical results indicated the retrofitted wall shear capacity for the gravity loads of 0.5 and 1MPa was 80% and 30%, respectively higher than that of the un-retrofitted wall. This value for the double-side retrofitting was 170% and 70%, respectively. It can be interpreted that with increasing gravity load, the effect of ECC mortar on the retrofitted wall lateral capacity decreased. According to the numerical results, the retrofitted wall shear strength under the gravity load of 0.1MPa and for ECC laxyer thicknesses of 10, 15, 20 and 30mm, were 140%, 180%, 230% and 350%, respectively more than that of the reference wall. It means that as the ECC laxyer thickness increased, the retrofitted wall shear strength increased. baxsed on the obtained results, a simple method for estimating the lateral strengths of retrofitted masonry walls with ECC mortar was presented. The experimental and numerical results showed that the use of ECC materials can significantly improve the in-plane behavior of undamaged and damaged URM walls, especially in increasing their shear strength and energy dissipation.