Abstract: In this paper the conversion ralation of the dynamic behavior (the natural vibration periods. the particapation factors, the maximum seismic base shear and the maximum seismic stresses) of geom etrically similar structures is deduced.It is concluded that the similar ratio 位 is the most important parameter in discussion of the influence of higher modes.For geometrically similar structures, the difference of its absolute sizes is greater, the difference of influence of higher modes is also greater. Therefore, the conclusion about the influence of higher modes for a certain structure is not always held for another similar structures in general.