The main purpose of this study is to obtain the optimum design of plan irregularity RC buildings in terms of their performance under static and dynamic loading. Torsional irregularity in RC structures is usually caused by the unsymmetrical form of the plan geometry or the stiffness distribution of the vertical elements. Structural optimization techniques should solve the problem to efficiently distribute materials throughout the structure in order to limit the torsional irregularity ratio at each storey layout. Different discrete mathematical models of 3D multi storey buildings were used for this study which is computationally efficient in solving such type of optimization problems. It is clearly seen that the results are changed considerably with the applying sizing optimization. The Turkish Earthquake Code TEC-2007 is used to analyse buildings and as constraint. ETABS program is used for analysis of 3D structural. The sizing optimization problem was formulated and implemented using MATLAB by employing Genetic Algorithm method. The design variables are constituted by the columns and shear walls of the structure which located at each storey layout.