Usage of moment frame systems as the seismic resistance system in steel building is increased in recent years. Because of proper seismic behavior and high energy-absorbing capacity in earthquake, evaluation of seismic behavior of these systems is noticed by researchers. Using of this system singly in tall buildings is not satisfied the seismic specifications of designed regulation because of large displacements. Combination of bracing in this system is proposed as the dual system. One of the models of combined systems that are used as the seismic resistant system in tall buildings in recent years is the moment frame and large scale bracing. In this paper, nonlinear behavior of this system that introduced as mega bracing system is assessed and is compared with the dual system of special moment frame and Xbracing. A frame with four floors is modeled in three statues mega bracing, X-bracing in external bays and X-bracing in internal bays. Area of braces is considered same in all systems. For evaluation of performance of this systems nonlinear statistical analysis and increment dynamic analysis was used. Results of analysis showed that of the ductility coefficient of mega bracing system is toward more than the combined system of X-bracing.