Practical, technical and economical difficulties in adopting base isolation created interest in study of mid-storey isolation, in which flexible isolators are installed at any intermediate storey of a building. Since vertical irregularities are increasing in the present architecture, a study to understand structural behaviour of these buildings under seismic loading is essential for proper design and better performance. This paper presents the time-history analysis of a stiffness and mass irregular building of G+20 stories and their seismic responses when incorporated with isolators at intermediate stories. The effect of change in isolation level in the seismic performance of the building is studied. The structural analysis software SAP2000 is used for the analysis process. Base shear, storey shear deviation and energy absorption of the high rise structures are plotted.