Generally, there are two methods for confrontation with earthquake force. The first method is to increase the seismic capacity of structure, for example, using lateral resistant systems such as moment frames, braced frames, shear walls and etc. The second method is to reduce the seismic requirement of structure using seismic base isolator systems. This paper, studies the effect of increase of period (as a result of reduction of the effective stiffness of seismic isolator) of seismic base isolator system of LRB type on the seismic response of three-dimensional steel structural models with concentrically braced frame system. For this purpose, three, six and ten story buildings have been modeled in both  states with fixed base and isolated base, along with different period and effective stiffness of isolator system in SAP2000 Software and have been investigated under dynamic spectral analysis (supposing that UBC97 code spectrum based on CAD and CVD). These buildings located in Zone 3 (seismic zone coefficient equal to Z= 0.3) and on Seismic Source Type “B” and SD soil type.  The results of analysis, including period, modal participating mass coefficient, stories shear and base share, have been investigated and compared.