Base isolation is one of the most widely accepted techniques to protect structures and to mitigate the risk to life and property from strong earthquakes. In this paper, the effect of mass asymmetry in symmetric building with unidirectional and bidirectional eccentricities and asymmetric building with upward and downward eccentricities in elevation is studied. To study the effect of torsion in seismic behavior of base isolated structures, a symmetric and asymmetric thirteen story concrete building is chosen as reference model. These models with mass eccentricity of 5%, 10%, 15% and 20% of greatest dimension of building in unidirection and bidirection are considered. The response spectrum and linear time history analysis of this eccentric model of fixed base and base isolated building using ETABS software is done. The symmetric and asymmetric fixed base and base isolated building having eccentricity in plan and elevation are compared for parameters such as maximum shear force, torsion, bending moment, lateral displacement, storey drift, storey acceleration and base shear. It is concluded that base isolation is very effective for various mass eccentric models. Response of base isolated mass eccentric model even in large mass eccentricity reduces as compared to fixed base building.