This paper describes a newly developed controllable damper using liquid crystal (LC) as a working fluid. LCs are homogeneous organic liquids characterized by the long-range order of their molecular orientation. The sample LC used in the present experiment is a thermotropic LC whose phase appears in nematic phase, originally developed for displays. The nematic phase is characterized by the molecular orientation of rod like molecules whose direction can be controlled by the applied electric or magnetic field. When an electric field is applied to a LC, it is known that the Orientation of the molecule becomes parallel to the applied electric field, which causes the viscosity increase in the flow perpendicular to the applied electric field. This phenomenon is defined as the electroviscous effect. As an application study of the electroviscous effect of LCs to controllable mechanical devices, a prototype controllable damper was constructed and its performance was examined. The performance was studied under various applied DC electric field strengths, piston amplitudes and frequencies. The results show that the controllable damping force was three times larger with electric field than that without electric field, and that the damping controllability was kept at the same level regardless of the frequency and amplitude of the piston reciprocation.