Earthquakes are one of the natural disasters that occur due to the sudden violent movement of earth's surface which releases energy and has high destructive power which can cause significant loss of life. On the other hand, earthquakes cause severe damage to property, especially to man-made structures and it provides architects and engineers with a number of important design criteria foreign to the normal design process. As many researchers studied and reviewed that seismic isolation is the best solution for the earthquake resistant of the structures and its application is the most constructive technique to protect structures against destruction from the earthquake strike and has gained growing majority during past decades. This is because base isolated confines the effects of the earthquake strike, a workable base largely separating the structure from the ground motion, and the structural counter accelerations are usually less as compared to the ground acceleration. The aim of the present study is to know the effect of the Lead rubber bearing base isolator on the seismic behaviour of plan irregular RC buildings i.e., (L Shaped, T Shaped and U Shaped) with different building heights. Static Pushover Analysis has been carried out using SAP2000 version 17 to know the seismic behaviour of buildings with and without Base Isolator.