With development of new seismic design concepts like performance based design, rehabilitation of existing reinforced concrete structures is of great importance nowadays, because they do not satisfy recent design criteria considering optimum performance level with providing both ductility and strength capacities in earthquakes with different hazard levels. Since adding Reinforced Concrete (RC) infills to RC frames is an effective rehabilitation procedure and openings in RC infills are inevitable, behavior study of RC frames strengthened by RC infills with openings seems necessary. In this paper, nonlinear behavior study of nine five-story concrete frames strengthened by RC infills with openings was done by performing nonlinear static analysis (Push Over), for which appropriate nonlinear modeling of system in PERFORM-3D software was verified by comparing results of a single story RC partially infilled frames under cyclic loading with experimental results of a similar existing study.Infill span, position and lateral load pattern affects strength, stiffness and ductility of a typical existing RC frame as behavior parameters. Adding RC infills obviously increases strength and stiffness of a RC frame, although it results in ductility decrease which can be prevented by proper positioning of infills. As a result, infill span is the most effective parameter on system strength and triangular lateral load pattern induce more ductility and strength capacities in system. Therefore; adding RC infills in proper positions is a way to enhance strength and stiffness of RC frames significantly with provision of appropriate ductility. As an example, a frame which is strengthened by infills with opening at the midspan shows better strength, stiffness and even ductility compared to those with openings near the columns.