Sliding Mode Control (SMC) has been used in many mechanical systems and structural system due to its robustness, simplicity and high control performance. However, choosing optimum parameters for systems is still an important research area. This study presents a numerical analysis to decrease the effect of earthquake vibrations on building model having Active Tuned Mass Damper (ATMD). The system is excited by an earthquake and a linear motor is used as the control device. ATMD is installed on top floor of building model. Tuning of Sliding Mode Controller (SMC) with Genetic Algorithm (GA) is designed for a three storey building model with ATMD. SMC parameters have been chosen by GA with a single objective function and a multiple objective function. Then, simulation results of uncontrolled and controlled model are compared. The results show that building model with SMC tuned by GA is effective to decrease the effects of earthquakes.