Real accelerations and subsequent wall diaplecements appear as the main damage Factors in liquid storage tanks. Base-isolation systems, as the most appropriate earthquake engineering options in passive control, can be used to reduce these factors and following destruction of these special structures. In this paper, the dynamic behavior of base-isolated rectangular tanks is studied under different ground motions in which, two tanks (one broad and one slender) with fixed-base and LRB base-isolatiors including different stiffnesses, i.e. stiff and flexible, are considered. In the developed model, finite elements and boundary elements for the shell and liquid domains and nonlinear hysteretic elements for isolation system are respectively employed to perform the analysis of the flexible tank in frequency and time domains. Further, the effect of base-isolators on the seismic response of the whole system including natural frequency, liquid sloshing height, base shear and moment and dynamic pressure on the tank shell, under harmonic and real earthquake has been inspected.