This paper deals with seismic characteristics of the single-stage bellows of Ti-Ni shape memory alloys, which are proposed as a new type of seismic elements. The single-stage bellows of different shapes are formed with the rubber bulge method and prepared for the dynamic tests. Tension-compression cyclic tests are carried out with increasing the displacement amplitude for them. Their effective stiffness and equivalent damping are standardized and estimated from the cyclic loops. Then the effects of bellows shape on dynamic properties are clarified. It is showed that nominal secant stiffness (effective stiffness) is strongly associated with shape factor H/Lw (H: bulge height, L_w: bulge width) of a single-stage bellows and that it can be expressed as a function of the coefficient of stiffness concerning the bulge shape and the material's stress strain characteristic.