Abstarct: The baxse isolation is one of methods to the resistance under earthquake forces. This method , lengthens the period of structure and reduces the responses of structure such as : baxse shear , story acceleration and story drift. Due to the importance of the behavior of these systems , under seismic loading , in this research , the behavior of a 5-story structure , have been evaluated by using three types of the elastomeric isolator with bilinear model that have different periods and dampings. For this purpose have been used of two earthquake recordes , Northridge and Tabas. For demonstration the effectiveness period of superstructure on baxse isolation have been used three periods for superstructure. The results indicate that baxse isolation , strongly reduses the responses of structure. In addition with lengthening the period of super structure , reduced the effectiveness of baxse isolation.