A shear-panel-damper, consists of a panel, surrounding four flanges and stiffeners, is used as an aseismic hysteretic damper for buildings in Japan. A crack easily grows in the shear-panel-damper when the panel shows shear buckling during cyclic loading under severe earthquake. The shear buckling can be detected only by checking the maximum-deformation- angle. For relatively thin panel, when width-to-thickness ratio is large, plastic deformation capacity of the damper can be evaluated by using the maximum-deformation-angle. On the contrary, when width-to-thickness ratio is relatively small, very low cycle fatigue life is essential for relatively thick panel to predict the usage limit of the damper because failure pattern change where crack grows the tow of welding between panel and flanges. Estimate expression of plastic deformation capacity for shear-thick-panel-damper, which has parameters as normalized width-to-thickness-ratio and deformation-angel was presented. To show the validity of the estimation expression, cyclic loading tests of shear-panel-damper were performed to verify the expression. Also clumped panels subjected to cyclic in-plane shear deformation were analyzed using geometrical and material non-linear F. E. M. analysis to check shear buckling of the panel. These results showed the validity and effectiveness of the expression.