In this paper, the seismic behaviour of mass isolated structures against the earthquake is studied. These structures are assumed to be consisted of two subsystems. Mass subsystem possesses low lateral stiffness but carries the major part of mass of the system. Stiffness subsystem, however, controls the deformation of the mass subsystem and attributes with much higher stiffness. The isolator layer is, therefore, located in between the mass and the stiffness subsystems and assumed to be a semi-active damper layer. The analytical model used for this investigation is a dual mass-spring model which is an extended form of the three-element Maxwell model. In this study, the ability of mass isolation techniques in reducing earthquake effects on buildings with two approaches, parametric and numerical approaches, is studied. In the parametric approach, by defining an isolation factor for structure and determining the dynamic characteristics of system, the relative optimum value of the isolator damping coefficient is obtained. Finally, in the numerical approach, the spectral responses of these structures due to the earthquake are investigated. The results show a noticeable decrease in earthquake input force to mass isolated structures in comparison with non-isolated structures and also show a suitable performance in control case.