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The mechanical properties of rubber that is the main member of a seismic isolation rubber bearing is evaluated for the cyclic tension, shear and torsion loading. The dimension of a specimen is a small size disk that imitates the bearing. The size and distribution of which the voids are nucleated inside rubbers applying tensile load depend on the rubber thickness. The preserving maximum shear stress rate (PMSSR) is defined as the shear stress at reaching the applying strain, 2 or 4 on each cycle divided by the shear stress on first cycle. The PMSSR on applying strain = 4 reduces with the increase of cycle numbers and more than on strain = 2. The tendency of reducing PMSSR depends on the thickness of rubber and the reduction attains about 20% at 30 cycles with more thickness than three millimeter and the reduction is enhanced by the voids inside rubber made by pre-tension. These results indicate that the assessment of durability will be needed for a post-seismic bearing.

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