There are several ways to control structures. ‌Base Isolateion is one of the most effective structure control systems for retrofitting low-rise buildings against earthquakes. When the number of story level of structure is increased, the base isolation performance decreases in reducing the structure response in upper stories. In this study, we are modelling a 20-story steel structure with a base isolation and use a Tuned Mass Damper to improve the performance of the isolated structure. The structure is modelled in 3D shape by SAP2000 software and a nonlinear time history analysis has been performed under 7 accelerations of the far field in the structure. The effect of installing mass damper is investigated on a structure equipped with a lead rubber bearing. Structural responses including the mean relative drift of the floors, the displacement of the roof and the absolute acceleration of the structure equipped with seismic isolation and mass damper is investigated. The results shows that the response of the isolated structure especially in its upper stories improves with combination of a mass damper and base isolation.