Although the reinforced concrete frame-infill systems are used throughout the world, they are rarely included in the numerical analysis of the structures. In this study, the effect of the infill walls on the natural period is investigated. Equivalent strut methodology is used to model the behavior of infill walls taking into consideration the effect of openings. A six floors reinforced concrete moment resisting frame building is modeled. Several configurations of infill walls are studied considering the wall openings. Natural vibration analysis is carried out for the studied building over a wide range of infill wall stiffness. The study is extended to investigate the interaction between the masonry infill walls and the R.C shear walls in buildings with different heights represented by the number of stories which ranges between 6 and 20 floors. All the considered shear wall buildings have fixed story total shear wall lengths to height ratios in each orthogonal direction. Finally, this study is intended with investigating the parameters of the equations presented by famous different codes to calculate the period of the shear wall buildings in comparison with modal analysis. It is found that excluding the effect of infill walls from modal analysis yields an undesired high overestimation of natural period increases as the height of building decreases. There is an extreme unrealistic variation in the results of natural period obtained from the second alternative of 2003 ECOL while there is high correlation between the results of modal analysis of wall infilled buildings and some specified code equations.