A series of unreinforced masonry (URM) walls were analytically investigated by FEM for a limited version of seismic in-plane performance. For this URM walls were assumed to be continuum and modeled as isotropic plane stress elements within which the nature of cracking was propagated. Accordingly, cracking mode of behavior in URM was modeled by smeared-crack approach. Total of 70 cases were considered for various parameters such as axial load ratio, aspect ratio and effective section area ratio due to the existence of opening. The analytical results indicate that these parameters significantly and interactively influence the ultimate strength of URM walls. Finally, it is suggested that the response modification factor for URM adopted in the current Korean Standard should be validated considering various forms of brittleness and probable failure modes in URM.