Abstract: Earthquake actions and seismic effects are studied for base-isolated cantilever beam. Based on the assumption that the dynamic characteristics of cantilever beams, with bending deformation curve of lateral displacement when subjected to lateral forces, are similar to those of high-rise structures, the base-isolated cantilever beam is investigated in this study as an equivalent simplified model of high-rise structures. Vibration periods and modes are derived from the Bernoulli-Euler theory, and earthquake responses are calculated with the mode decomposition response spectrum method. The seismic modal effects of pre- and post- isolation are then examined. It is found that seismic-effect adjustment of each mode is the main mechanism of seismic reduction of isolated high-rise structures and that the higher modes contribution is reduced with periods of the higher modes elongated. Other findings include that base-isolation influence on the substructure is different from the upper-structure, that earthquake-response variation of shear is more significant than that of the moment, and that the reduction of the higher modes is more significant with lower isolation stiffness. It is suggested that the fundamental period ratio of the post- and pre- isolation be over 1.4, and that seismic effect of base-isolated regular high-rise structure can be calculated with two lowest modes alone.