Most structures follow nonlinear behavior under sever earthquake loading conditions. In other words, stress and strain in most elements lie outside the elastic boundaries of the materials that the elements are consist of. In these cases, plastic hinges should have appropriate ductility in order for the structure to have a more desirable behavior until arrival in the ultimate displacement after entering the plastic regime. For this purpose, many methods have been presented to increase ductility capacity of beams, columns, and joints. Another approach, however, may involve decreasing ductility demand of the above elements with the aid of base isolation concept. By increasing the natural period of the structure the base-isolation system moves the structure natural frequency farther from the earthquake's high-energy frequency interval, in other words, the transferred forces to the structure caused by the earthquake will decrease with the isolation of structure from the earth. In this thesis, the base isolation system effect on the response of several RC structures is shown. This requires that concepts such as nonlinear time-history analysis, plastic hinges, ductility capacity of RC structures, and base isolation be thoroughly discussed. As a result, the structure response will be available via nonlinear time-history analysis. From amongst most important parameters that are considered in the analysis of structure response under earthquake loading, we can name relative displacement of stories, base shear, and joint rotation. Increase in any of such parameters can facilitate entrance of structural elements into the plastic regime. The investigation of any of the above parameters can determine the amount of increase or decrease in the required structural ductility.