Because a base isolated structure is usually a long-period dynamic system, an excessive response can be easily induced by a near-fault earthquake that possesses a long-period pulse-like waveform. To alleviate this problem, a rocking bearing that has a variable isolation frequency is proposed in this study. The proposed rocking bearing has an articular (ball-and-socket) joint on the top and a rocking surface with a variable curvature on the lower part. The articular joint is connected to the super-structure through a mounting plate. In an earthquake, the rocking surface of the bearing will rock back-and-forth on a base plate that is mounted on the ground or the foundation of the structure. By properly selecting the bearing's geometric parameters, the isolation stiffness of the proposed rocking bearing can be a function of the bearing displacement. As a result, the bearing's isolation frequency becomes variable and can be determined by the geometric parameters, exclusively. In this study, the bearing's rocking surface is defined by a six-order polynomial function, so the restoring force of the bearing exhibits a softening behavior followed by a hardening behavior. This mechanical behavior aims to suppress the maximum structural acceleration and isolator displacement, simultaneously. By comparing with the response of a conventional FPS isolation system, the isolation performance of the rocking bearing with its rocking surface defined by the six-order polynomial is investigated numerically in this study.